Driver circuit, signal processing unit having the driver circuit, method for manufacturing the signal processing unit, and display device

ABSTRACT

Disclosed is a driver circuit including a latch circuit, a shift register circuit, and a switching circuit, where the latch circuit is provided over the shift register circuit and the switching circuit. The shift register circuit and the switching circuit may have a silicon-based semiconductor, while the latch circuit may have an oxide semiconductor. The latch circuit includes a first transistor and a second transistor connected in series. The latch circuit may further include a first capacitor and a second capacitor which are electrically connected to the first transistor and the second transistor. A display device using the driver circuit as well as a method for preparing the driver circuit is also disclosed.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor device, a displaydevice, a light-emitting device, a driving method thereof, or amanufacturing method thereof. The present invention also relates to adriver circuit, a signal processing unit having the driver circuit, amethod of manufacturing the signal processing unit, and a displaydevice.

2. Description of the Related Art

Driving methods of display devices such as liquid crystal displaydevices and electroluminescent (EL) display devices can be broadlyclassified into a passive matrix method and an active matrix method. Anactive matrix method has been widely employed for pixel portions oftelevision receivers, cellular phones, and the like because it canachieve low power consumption, high definition, enlargement ofsubstrates, and the like as compared with a passive matrix method.

Along with the increase in size and definition of panels employingactive matrix driving, development of performance of driver circuits forcontrolling driving of pixel portions has been required. For example, atechnique has been employed in which a high-performance semiconductorintegrated circuit manufactured using a material having very highmobility, such as a single crystal silicon substrate, is mounted, as adriver circuit, on a display device by a chip on glass (COG) method or atape automated bonding (TAB) method (e.g., see Patent Document 1 for theCOG method).

A semiconductor integrated circuit used as a driver circuit is referredto as a signal processing unit, a display signal processing unit, adisplay driver, or the like in general, but in this specification,referred to as a signal processing unit. Further, a circuit t in thesignal processing unit, which relates to driving of a display device, isreferred to as a driver circuit.

REFERENCE

-   Patent Document 1: Japanese Published Patent Application No.    2003-255386

SUMMARY OF THE INVENTION

In recent years, with the widespread use of tablet computers,smartphones, and the like, the definition of pixel portions has beensignificantly increased and accordingly the power consumption of signalprocessing units has also been increased. It is known that an importantfactor for comparing the performances of tablet computers andsmartphones is how long they can operate after a single charge (alsoreferred to as operation time, driving time, or the like). Thus, drivercircuits included in a signal processing unit have been expected to havelower power consumption.

Further, there have been heavy demands for a display device that can beprovided on a curved surface, a display device that can be freelydeformed by a stress applied in bending or twisting, and the like, whichhave motivated intensive research on flexible display devices. However,since the flexibility of a signal processing unit using a single crystalsilicon substrate or the like as described above is low, when a stresssuch as a bend or a twist is applied to the display device, a problemsuch as peeling of the mounted signal processing unit or a breakdownthereof due to a shock might be caused.

In view of the above problem, an object of the present invention is toprovide a driver circuit having low power consumption.

Another object of the present invention is to provide a signalprocessing unit which includes the above driver circuit, has low powerconsumption, and hardly causes a breakdown or a separation due toapplication of a stress.

Another object of the present invention is to provide a method ofmanufacturing the above signal processing unit.

Another object of the present invention is to provide a display deviceincluding the above signal processing unit.

Note that all of the embodiments of the present invention do not need toachieve all the objects. Other objects can be derived from thedescription of the specification, the drawings, and the claims.

A driver circuit included in a signal processing unit usually includes ashift register circuit, a latch circuit, and a switching circuit. Ingeneral, operations of the driver circuit are successively performed foreach output line (also referred to as each output stage) as follows: (1)A signal (pulse signal) which determines, among a plurality of outputlines connected to the driver circuit, an output line to which data isoutput is output from a shift register circuit. (2) When the signal fromthe shift register circuit is input to the latch circuit to which animage signal (also referred to as a video signal, video data, or thelike) is input, analog data representing the gray level of the videosignal is output. (3) The analog data output from the latch circuit isoutput to the output line via the switching circuit. Note that theswitching circuit may precharge data (also can be referred to as charge)accumulated in the latch circuit.

The latch circuit successively outputs the same data to the switchingcircuit until a different signal is input from the shift registercircuit. Thus, even when sequential output signals are the same as eachother without change in the video signal input to the latch circuit (forexample, when a still image is displayed), the operation state continuesand electric power is consumed.

In view of the above, an embodiment of the present invention is a drivercircuit including at least a shift register circuit, a latch circuit,and a switching circuit, where the latch circuit has either of thefollowing structures: (1) a structure including a first input terminal,a second input terminal, an output terminal, a capacitor, and atransistor including an oxide semiconductor film as an active layer(hereinafter, referred to as an OS transistor in some cases); and (2) astructure including a first input terminal, a second input terminal, anoutput terminal, a transmission gate, an inverter, a clocked inverter,and an OS transistor.

The energy gap of an oxide semiconductor used for a semiconductor layerof the OS transistor is greater than or equal to 3.0 eV, which is muchlarger than the band gap of silicon (1.1 eV).

The off-resistance of the transistor (resistance between source anddrain when the transistor is in an off-state) is inversely proportionalto the concentration of carriers thermally excited in the semiconductorlayer where a channel is formed. Since the band gap of silicon is 1.1 eVeven in a state where there is no carrier caused by a donor or anacceptor (i.e., even in the case of an intrinsic semiconductor), theconcentration of thermally excited carriers at room temperature (300 K)is approximately 1×10¹¹ cm⁻³.

In contrast, in the case of a semiconductor whose band gap is 3.2 eV (anoxide semiconductor, here), the concentration of thermally excitedcarriers is approximately 1×10⁻⁷ cm⁻³. When the electron mobility is thesame, the resistivity is inversely proportional to the carrierconcentration, so that the resistivity of the semiconductor whose bandgap is 3.2 eV is 18 orders of magnitude higher than that of silicon.

Such an OS transistor in which an oxide semiconductor having a wide bandgap is used for the semiconductor layer can achieve an extremely lowoff-state current. Accordingly, in the above structures (1) and (2),data (image data) input to the latch circuit can be held even when thelatch circuit is off. Thus, particularly when the same data (image data)is successively input to the latch circuit, for example, when a stillimage or a slow moving image is displayed, the power consumption of thedriver circuit can be reduced.

Note that as the latch circuit of an embodiment of the invention, alatch circuit having a configuration illustrated in FIG. 17A, forexample, can be formed.

A latch circuit 1000 illustrated in FIG. 17A includes an OS transistor1002. One of a source and a drain of the transistor 1002 is electricallyconnected to a first input side (In_1), a gate is electrically connectedto a second input side (In_2), and the other of the source and drain iselectrically connected to an output side (Out). Note that the followingexplanation is given assuming that the output side (Out) is in afloating state.

In the latch circuit 1000, when the transistor 1002 is turned off by asignal input from the second input side (In_2), data is held in a node1003 electrically connected to the other of the source and drain of thetransistor 1002 and to the output side (Out).

The OS transistor features an extremely low off-state current (a currentflowing between the source and the drain when the transistor is off) inan off state (i.e., the state where the gate voltage is lower than thethreshold voltage in the case of an n-channel transistor, or the statewhere the gate voltage is higher than the threshold voltage in the caseof a p-channel transistor). Accordingly, there is no possibility or anextremely small possibility of leakage of the data held in the node 1003to the one of the source and drain from the other of the source anddrain (in FIG. 17A, the heavy line arrow indicates the concept and the xmark indicates no leakage of the data). The data can thus be held in thenode 1003.

Hence, in the case where a signal is input to the latch circuit andaccumulated in the node 1003 and then a signal next input to the latchcircuit is the same as the previously input signal, the previously inputsignal is held in the node 1003 and output from the latch circuit 1000even when the transistor 1002 is off (the transistor 1002 is in annon-operation state). Therefore a signal does not necessarily need to beinput to the first input side (In_1) or the second input side (In_2).Accordingly, power consumption in the latch circuit can be reduced andlow power consumption of the driver circuit can be achieved.

Although one latch circuit is given for easier understanding in theabove explanation, a structure using two latch circuits (the latchcircuit 1000 a and the latch circuit 1000 b) connected in series asillustrated in FIG. 17B can be employed.

In this structure, first, a transistor 1002 a is turned on byapplication of a voltage from the second input side (In_2) and datainput from the first input side (In_1) is accumulated in a node 1003 a.Then, the transistor 1002 a is turned off, so that the data is held inthe node 1003 a.

Next, a transistor 1002 b is turned on by application of a voltage froma third output side (In_3) of the transistor 1002 b and the data (alsoreferred to as charge) in the node 1003 a is accumulated in a node 1003b. Then, the transistor 1002 b is turned off, so that the data is heldin the node 1003 b.

Since the transistors 1002 a and 1002 b are both OS transistors, leakageof the data held in the node 1003 b does not occur as long as the outputside (Out) is in a floating state. In addition, because the node 1003 ais electrically connected to the other of a source and a drain of thetransistor 1002 a and one of a source and a drain of the transistor 1002b, leakage of the data held in the node 1003 a does not occur. Thus,even when the transistors 1002 a and 1002 b are turned off, the dataheld in the node 1003 b is output from the output side (Out).

However, in the case of a general latch circuit (i.e., a latch circuitnot including an OS transistor as a component), when the same signal issuccessively input to the latch circuit, an unexpected signal might beoutput from an output side unless the latch circuit is constantly on(i.e., a transistor included in the latch circuit is constantly on).

In the case of a latch circuit having the above-described structureusing an OS transistor, when the same signal is successively input tothe latch circuit from the first input side (In_1), once a signal firstinput is held in the nodes 1003 a and 1003 b, data output from theoutput side does not vary even if the latch circuit is off (that is, thetransistors 1002 a and 1002 b are off). Thus, particularly when the samesignal is successively input to the latch circuit, power consumption inthe latch circuit can be reduced and low power consumption of the drivercircuit can be achieved.

Note that the expression “the same signal is successively input” meansthat the same signal is input successively twice or more.

Further, for transistors included in the shift register circuit and theswitching circuit, the use of a single crystal semiconductor film as anactive layer enables high-speed operation. In the driver circuit, whilea single crystal semiconductor film is used as an active layer in thetransistor included in a component that needs to perform high-speedoperation, an oxide semiconductor film is used as an active layer atleast in a transistor included in the latch circuit, so that the powerconsumption of the latch circuit can be reduced and the driver circuitcan achieve low power consumption particularly when the same signal issuccessively input to the latch circuit.

In an embodiment of the present invention, the OS transistor included inthe latch circuit is provided in a layer different from a layer in whichthe transistors (transistors each including a single crystalsemiconductor film as an active layer) included in the shift registercircuit and the switching circuit are provided. Note that hereinafter, alayer including a transistor including a single crystal semiconductorfilm as an active layer is referred to as a first layer and a layerincluding an OS transistor is referred to as a second layer. Further,the driver circuit is formed by the first layer and the second layer,and a layer which collectively includes the first layer and the secondlayer is referred to as a circuit layer.

With the driver circuit having the above structure, low powerconsumption of the driver circuit can be achieved.

With a structure in which the above-described driver circuit is providedover a flexible substrate, it is possible to provide a signal processingunit which hardly causes a breakdown or a peeling due to a shock orapplication of a stress in bending or twisting. Note that, in thespecification, a flexible substrate is a substrate which can be deformedin accordance with a form of an object to be bonded, and an insulatingresin material, a metal material with an insulating surface, and thelike can be used as the flexible substrate.

The above signal processing unit included in a circuit layer is bondedto (mounted on) a substrate including a pixel portion, and the drivercircuit and the pixel portion are electrically connected to each otherto operate the pixel portion (i.e., the driver circuit in the signalprocessing unit is used as a scan line driver circuit or a signal linedriver circuit). Thus a display device having low power consumption canbe achieved. In addition, a flexible substrate can be used in the signalprocessing unit, and accordingly the thickness or weight of the displaydevice can be reduced.

In addition, the substrate including the pixel portion may haveflexibility. If a flexible substrate is also used for the signalprocessing unit, a problem such as a peeling of the mounted signalprocessing unit or a breakdown of the signal processing unit itself canbe prevented.

That is, one embodiment of the present invention is a driver circuitincluding a latch circuit configured to hold data, a shift registercircuit configured to output a signal capable of determining anoperation state of the latch circuit, and a switching circuit configuredto determine whether a signal output from the latch circuit is output tothe outside or not. The latch circuit includes a first input terminal, asecond input terminal, an output terminal, and a transistor including anoxide semiconductor film as an active layer. A gate of the transistor iselectrically connected to the first input terminal. One of a source anda drain of the transistor is electrically connected to the second inputterminal. The other of the source and the drain of the transistor iselectrically connected to the output terminal. Data is held in a nodeelectrically connected to the other of the source and the drain of thetransistor and to the output terminal, when the transistor is off. Atransistor included in the shift register circuit and a transistorincluded in the switching circuit are each a transistor including asingle crystal semiconductor film as an active layer. Note that thelatch circuit may further include a capacitor. In this case, the otherof the source and the drain of the transistor is electrically connectedto one of electrodes of the capacitor, and the other of the electrodesof the capacitor is electrically connected to a wiring to which a fixedpotential is supplied.

With the driver circuit having the above structure, the driver circuitcan achieve low power consumption.

The structure of the above latch circuit may be as follows. The latchcircuit includes a first input terminal, a second input terminal, anoutput terminal, a transmission gate, an inverter, a clocked inverter,and a transistor including an oxide semiconductor film as an activelayer. An input terminal of the transmission gate is electricallyconnected to the first input terminal, and an output terminal of thetransmission gate is electrically connected to an input terminal of theinverter and an output terminal of the clocked inverter. An outputterminal of the inverter is electrically connected to one of a sourceand a drain of the transistor and to the output terminal. The other ofthe source and the drain of the transistor is electrically connected toan input terminal of the clocked inverter. Data is held in a nodeelectrically connected to the output terminal of the inverter, the oneof the source and the drain of the transistor, and the output terminal,when the transistor is turned off. A transistor included in thetransmission gate, a transistor included in the inverter, and atransistor included in the clocked inverter may include a single crystalsemiconductor film as an active layer.

Another embodiment of the present invention is a signal processing unitincluding a base substrate and a circuit layer including the abovedriver circuit over the base substrate. The circuit layer includes atleast a first layer in which the transistor including a single crystalsemiconductor film as an active layer is formed and a second layer inwhich the transistor including an oxide semiconductor film as an activelayer is formed. Note that, as the base substrate, a flexible substratecan be used.

With the signal processing unit having the above structure, the signalprocessing unit can achieve low power consumption and hardly causes abreakdown or a peeling due to application of a stress.

As the single crystal semiconductor film included as the active layer ofthe above transistor, a film obtained by separation of part of a singlecrystal semiconductor substrate is preferably used. The separation ofthe film can be performed at a relatively low temperature (e.g., atemperature lower than or equal to 600° C.) and enables a single crystalsemiconductor film to be formed even over a large area substrate, whichleads to an improvement in the productivity or a cost reduction of thedriver circuit and the signal processing unit.

Another embodiment of the present invention is a display deviceincluding a substrate, the aforementioned signal processing unit overthe substrate, a counter substrate bonded to the substrate with asealing material, and a display element layer between the substrate andthe counter substrate. The driver circuit in the signal processing unitis electrically connected to the pixel portion. The signal processingunit functions as one or both of a scan line driver circuit and a signalline driver circuit.

With the display device having the above structure, the display devicecan achieve low power consumption. In addition, since the base substrateincluded in the signal processing unit can be a flexible substrate, thethickness and weight of the display device can be reduced.

By using a flexible substrate for the substrate in the above displaydevice, the display device can achieve low power consumption and hardlycauses a breakdown or a peeling due to application of a stress.

When the substrate partly has a depressed portion, the signal processingunit is provided in the depressed portion, and the driver circuit iselectrically connected to the pixel portion, so that the display devicecan be resistant to a shock from the outside and hardly causes abreakdown or a peeling due to application of a stress in bending ortwisting. In addition, when the signal processing unit is provided inthe depressed portion so that the base substrate covers the drivercircuit and that a bottom surface of the base substrate (a surface overwhich the circuit layer is not formed) is positioned in the same plan asa top surface of the substrate, the display device can be furtherresistant to a shock from the outside and a breakdown or a peeling dueto application of a stress such in bending or twisting.

When the base substrate has a larger area than the circuit layer and thepixel portion is covered with the base substrate, the counter substratecan be eliminated and the manufacturing process of the display devicecan be simplified.

Another embodiment of the present invention is a method of manufacturinga signal processing unit including a driver circuit including a latchcircuit, a shift register circuit, and a switching circuit, whichincludes the steps of: forming an embrittled region at a predetermineddepth of a single crystal semiconductor substrate having an insulatingsurface by irradiation of one side of the single crystal semiconductorsubstrate with one or both of a hydrogen ion and a rare gas ion; forminga separation layer over the side of the single crystal semiconductorsubstrate; bonding a first temporary fixing substrate to the singlecrystal semiconductor substrate with the separation layer interposedtherebetween; subjecting the single crystal semiconductor substrate toheat treatment; providing, over the first temporary fixing substrate, asingle crystal semiconductor thin film separated from the single crystalsemiconductor substrate along the embrittled region by separating thesingle crystal semiconductor substrate from the first temporary fixingsubstrate; forming a transistor by using the single crystalsemiconductor film as an active layer to form a first circuit layerincluding a shift register circuit and a switching circuit; forming thedriver circuit over the separation layer over the temporary fixingsubstrate by forming, over the first circuit layer, a second circuitlayer including a latch circuit having a transistor including an oxidesemiconductor film as an active layer; bonding a second temporary fixingsubstrate onto the second circuit layer with use of a temporary fixingmaterial; providing, over the second temporary fixing substrate, thefirst circuit layer and the second circuit layer separated from thefirst temporary fixing substrate at the separation layer by separatingthe second temporary fixing substrate from the first temporary fixingsubstrate; bonding, with use of a bond material, a flexible basesubstrate to a side of the first circuit layer where the secondtemporary fixing substrate is not formed; and providing the drivercircuit over the base substrate by separating the second temporaryfixing substrate from the base substrate at the temporary fixingmaterial. Note that a boding layer may be formed over the separationlayer.

By the above manufacturing method, a signal processing unit which haslow power consumption and hardly causes a breakdown or a peeling due toapplication of a stress can be manufactured.

In the above manufacturing method, the separation layer may be formedover the first temporary fixing substrate. Specifically, an embrittledregion may be formed in a single crystal semiconductor substrate havingan insulating surface by irradiation of one side of the single crystalsemiconductor substrate with one or both of a hydrogen ion and a raregas ion, and a first temporary fixing substrate whose surface isprovided with a separation layer may be bonded to the one side of thesingle crystal semiconductor substrate so that the one side of thesingle crystal semiconductor substrate is in contact with the separationlayer of the first temporary fixing substrate. By using this method,formation of the insulating film on the surface of the semiconductorsubstrate, formation of the embrittled region in the semiconductorsubstrate, and formation of the separation layer over the firsttemporary fixing substrate can be performed at the same time withdifferent apparatuses, which can shorten the time for manufacturing thesignal processing unit.

In the above method of manufacturing a signal processing unit, by usinga silicon substrate, a germanium substrate, a silicon germaniumsubstrate, a silicon carbide substrate, a gallium arsenide substrate, oran indium phosphide substrate as the first temporary fixing substrate,electrical characteristics (e.g., mobility) of a semiconductor elementincluding the transistor including a single crystal semiconductor filmas an active layer can be improved, which leads to higher performance ofthe signal processing unit.

In the above method of manufacturing a signal processing unit, bysubjecting the separation layer to planarization treatment, it ispossible to prevent a problem in which a part of or the whole of thesingle crystal semiconductor thin film cannot be transferred to thefirst temporary fixing substrate side due to a defect in bonding betweenthe separation layer and the first temporary fixing substrate.

Another embodiment of the present invention is a method of manufacturinga display device, which includes the steps of forming a signalprocessing unit by the above method and electrically connecting, with aconductive material, the signal processing unit to a pixel portionprovided between a substrate and a counter substrate that are bonded toeach other with a sealing material.

By the above manufacturing method, a display device that can achieve lowpower consumption can be manufactured.

In the above method of manufacturing a display device, a substrate whosesurface is provided with a depressed portion can be used. When and thedriver circuit and the pixel portion are electrically connected in thedepressed portion with the use of the conductive material, a displaydevice that can be resistant to a shock from the outside and hardlycauses a breakdown or a peeling due to application of a stress inbending or twisting can be manufactured.

The base substrate may be allowed to serve as the counter substrate inaddition to as the base substrate included in the signal processingunit, by which the manufacturing process of the display device can besimplified, so that time and cost for manufacturing the display devicecan be reduced. In the above method of manufacturing a display device,by using a flexible substrate for the substrate included in the displaydevice, a display device that can achieve low power consumption andhardly causes a breakdown or a peeling due to application of a stresscan be manufactured.

In an embodiment of the invention, the circuit layer which forms thedriver circuit has a stacked layer structure including at least twolayers, a first layer including a transistor including a single crystalsemiconductor film as an active layer and a second layer including atransistor including an oxide semiconductor film as an active layer. Thecircuit layer includes a shift register circuit, a latch circuit, and aswitching circuit, in which the latch circuit includes the transistorincluding an oxide semiconductor film as an active layer as a component.

Accordingly, a video signal input to the driver circuit can be held inthe latch circuit. When the same signal is successively input to thelatch circuit (for example, a still image is displayed), powerconsumption in the latch circuit can be reduced and the powerconsumption of the signal processing unit can be reduced particularlyeffectively.

Note that high-speed driving performance can also be ensured by usingthe transistor including a single crystal semiconductor film as anactive layer as a component included in the circuit layer other than thelatch circuit.

Further, when the signal processing unit is formed by using a flexiblesubstrate, a breakdown or a peeling hardly occurs even if the signalprocessing unit is provided in a portion having a curved surface or evenif a stress is applied in a bending or twisting.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIGS. 1A to 1C illustrate a configuration of a driver circuit includedin a circuit layer;

FIGS. 2A to 2D illustrate a method of manufacturing a signal processingunit;

FIGS. 3A to 3D illustrate a method of manufacturing a signal processingunit;

FIGS. 4A to 4D illustrate a method of manufacturing a signal processingunit;

FIGS. 5A and 5B illustrate a method of manufacturing a signal processingunit;

FIGS. 6A and 6B illustrate a method of manufacturing a signal processingunit;

FIG. 7 illustrates a method of manufacturing a signal processing unit;

FIGS. 8A to 8C illustrate a method of manufacturing a signal processingunit;

FIGS. 9A to 9C illustrate a configuration of a display device;

FIGS. 10A to 10D illustrate a method of manufacturing a display device;

FIGS. 11A to 11C illustrate a method of manufacturing a display device;

FIG. 12 illustrates a method of manufacturing a display device;

FIGS. 13A to 13C illustrate a method of manufacturing a display device;

FIGS. 14A to 14C illustrate a method of manufacturing a display device;

FIG. 15 illustrates a method of manufacturing a display device;

FIGS. 16A to 16F each illustrate a semiconductor device including asignal processing unit;

FIGS. 17A and 17B illustrate a configuration and operation concept of alatch circuit;

FIG. 18 shows characteristics of a transistor including an oxidesemiconductor;

FIG. 19 is a diagram of a circuit for evaluation of characteristics of atransistor including an oxide semiconductor;

FIG. 20 is a timing chart for evaluation of characteristics of atransistor including an oxide semiconductor;

FIG. 21 shows characteristics of a transistor including an oxidesemiconductor;

FIG. 22 shows characteristics of a transistor including an oxidesemiconductor;

FIG. 23 shows characteristics of a transistor including an oxidesemiconductor; and

FIGS. 24A to 24D illustrate a method of manufacturing a signalprocessing unit.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, embodiments of the invention disclosed in thisspecification will be described with reference to the accompanyingdrawings. Note that the present invention is not limited to thefollowing description and it will be readily appreciated by thoseskilled in the art that modes and details can be modified in variousways without departing from the spirit and the scope of the presentinvention. Thus, the present invention should not be construed as beinglimited to the description in the following embodiments.

In the following embodiments, the same portions or portions havingsimilar functions are denoted by the same reference numerals indifferent drawings, and explanation thereof will not be repeated.

Note that the position, size, range, and the like of each componentillustrated in the drawings and the like are not accurately representedfor easy understanding in some cases. Thus, the disclosed invention isnot necessarily limited to the position, size, range, and the like inthe drawings and the like.

In this specification and the like, ordinal numbers such as “first”,“second”, and “third” are used in order to avoid confusion amongcomponents, and the terms do not limit the components numerically.

In this specification and the like, the term such as “over” or “below”does not necessarily mean that a component is placed “directly on” or“directly under” another component. For example, the expression “B overA” can mean the case where there is an additional component between Aand B.

Further, in this specification and the like, the step of moving a filmformed over a substrate or part of a substrate to another substrate(i.e., the step of putting part of a film or a substrate formed over asubstrate in another place) is referred to as “transfer” or “a transferstep”.

Embodiment 1

In this embodiment, a configuration example of a signal processing unit,a circuit configuration example of a driver circuit provided in acircuit layer of the signal processing unit, and a configuration exampleof a latch circuit included in the driver circuit are described usingFIG. 1A, FIG. 1B, and FIG. 1C, respectively. Further, an example of amethod of manufacturing the signal processing unit is described usingFIGS. 2A to 2D, FIGS. 3A to 3D, FIGS. 4A to 4D, FIGS. 5A and 5B, FIGS.6A and 6B, FIG. 7, and FIGS. 8A to 8C.

<Structure Example of Driver Circuit>

As illustrated in FIG. 1A, a signal processing unit 100 has a structurein which a circuit layer 110 is formed over a base substrate 115. Thecircuit layer 110 includes a first layer 111 including a transistorincluding a single crystal semiconductor film as an active layer and asecond layer 112 including a transistor including an oxide semiconductorfilm as an active layer.

<Configuration Example of Circuit Layer>

FIG. 1B illustrates a circuit configuration example of a driver circuitprovided in the circuit layer 110 of the signal processing unit 100. Asillustrated in FIG. 1B, the driver circuit provided in the circuit layer110 includes a shift register circuit 102 including a plurality of (alsoreferred to as plural stages of) flip flops 101, a first latch circuitgroup 104 including a plurality of (also referred to as plural stagesof) first latch circuits 103, a second latch circuit group 106 includinga plurality of (also referred to as plural stages of) second latchcircuits 105, and a switching circuit 113.

The switching circuit 113 includes an analog switch 107 and a transistor108 at each stage. The analog switches 107 are connected so that outputsides of the stages of the second latch circuit group 106 are connectedor disconnected to signal lines 51 to Sn. Further, the transistors 108electrically connected to the analog switches 107 at the stages areconnected so that wirings 109 are connected or disconnected to thesignal lines S1 to Sn. Note that a potential to be a non-display signal(e.g., ground potential) input to each pixel is applied to the wiring109.

Note that the configuration of the switching circuit 113 is not limitedto the configuration illustrated in FIG. 1B and may be any configurationas long as either a display signal or a non-display signal is output tothe signal lines 51 to Sn.

In the above-described driver circuit, for example, a configurationillustrated in FIG. 1C can be employed for the first latch circuit 103and the second latch circuit 105. This configuration corresponds to astructure in which a capacitor 1008 a and a capacitor 1008 b areconnected to the node 1003 a and the node 1008 b of FIG. 17B,respectively. One of electrodes of the capacitor 1008 a is electricallyconnected to the other of the source and drain of the transistor 1002 aand one of the source and drain of the transistor 1002 b, and the otherof the electrodes is connected to a fixed potential (e.g., groundpotential). One of electrodes of the capacitor 1008 b is electricallyconnected to the other of the source and drain of the transistor 1002 band to the output side, and the other of the electrodes is connected toa fixed potential (e.g., ground potential). Note that video data(Video_Data) is input to one of the source and drain of the transistor1002 a, and a signal (FF_Signal) from the flip flop 101 is input to agate of the transistor 1002 a. A latch pulse (Latch_Pulse) is input to agate of the transistor 1002 b, and the output side (Out) of thetransistor 1002 b is electrically connected to the switching circuit113. Note that the first latch circuit 103 and the second latch circuit105 can have a structure in which one or both of the capacitors 1008 aand 1008 b is not provided.

<Explanation of Operation of Circuit Layer 110>

Next, operation of the driver circuit included in the circuit layer 110is described using FIG. 1B.

A clock signal (S_CLK), an inverted clock signal (S_CLKB), and a startpulse signal (S_SP) are input to the shift register circuit 102. Inaccordance with these signals, sampling pulses are sequentially outputfrom the shift register circuit 102.

The sampling pulses output from the shift register circuit 102 are inputto the first latch circuit 103, and video signals (Video Data) are heldin the first latch circuit 103 in accordance with the timing of thesampling pulse.

After holding of the video signals is completed up to the last stage inthe first latch circuit 103, latch pulses are input to the second latchcircuit 105 and the video signals held in the first latch circuit 103 ina horizontal retrace period are simultaneously transmitted to the secondlatch circuit 105.

The video signals transmitted to the second latch circuit 105 are inputto the switching circuit 113. A control signal (S_WE) is input to theswitching circuit 113. The control signal controls whether the signalsheld in the second latch circuit 105 or the signal which puts a pixel ina non-displaying state is supplied to the signal lines 51 to Sn. Thatis, when the control signal (S_WE) is at an H level, a non-displaysignal is output from the switching circuit 113 to a pixel, whereas whenthe control signal (S_WE) is at an L level, a video signal is outputfrom the switching circuit 113 to a pixel.

The circuit layer 110 has two latch circuits (the first latch circuit103 and the second latch circuit 105) at each stage in this embodimentbut is not limited to this structure and may have two or more latchcircuits at each stage.

The latch circuits described in this embodiment feature the transistorincluding an oxide semiconductor film as an active layer which is formedin the second layer 112.

<Explanation of Operation of First and Second Latch Circuits>

Operation flow of the first latch circuit 103 and the second latchcircuit 105 illustrated in FIG. 1C is briefly described.

First, a signal (FF_Signal) from the flip flop is input to the gate ofthe transistor 1002 a included in the first latch circuit 103, and thetransistor 1002 a is turned on by the signal. Accordingly, video data(Video_Data) is accumulated in the capacitor 1008 a and then thetransistor 1002 a is turned off.

Although the one of the electrodes of the capacitor 1008 a is connectedto the other of the source and drain of the transistor 1002 a and to theone of the source and drain of the transistor 1002 b, since thetransistors 1002 a and 1002 b are OS transistors and feature anextremely low off-state current, there is no possibility or an extremelysmall possibility of leakage of the data accumulated in the capacitor1008 a (leakage through the transistor 1002 a or the transistor 1002 b).Thus, the data accumulated in the capacitor 1008 a continues to be heldfor a long time; therefore, in the case where, for example, the samesignal is successively input to the first latch circuit 103, thetransistor 1002 a can be off once data is held in the capacitor 1008 a.

Next, a latch pulse (Latch_Pulse) is input to the gate of the transistor1002 b in the second latch circuit 105, and the transistor 1002 b isturned on by the signal. Accordingly, the data held in the capacitor1008 a is accumulated in the capacitor 1008 b, and then the transistor1002 b is turned off.

One of the electrodes of the capacitor 1008 b is connected to the otherof the source and drain of the transistor 1002 b and to the output side.Therefore, by putting the output side in a floating state, leakage ofthe data accumulated in the capacitor 1008 b (leakage through thetransistor 1002 b or leakage to the output side) can be suppressed.Accordingly, in the case where, for example, the same signal issuccessively input to the second latch circuit 105, the transistor 1002b can be off once data is held in the capacitor 1008 b.

Since a general latch circuit itself does not have a function of holdingdata for a long time, even when the same video data is successivelyinput to the latch circuit, data output from the output side changesunless the latch circuit is on. In contrast, by using the latch circuithaving the above-described structure, when the same video data issuccessively input to the latch circuit, once the data first input isheld in the capacitor 1008 a and the capacitor 1008 b, data output fromthe output side does not change even if the first latch circuit 103 andthe second latch circuit 105 are turned off (i.e., the signal input fromthe flip flop 101 or the latch pulse (Latch_Pulse) input to the secondlatch circuit 105 is stopped). In particular, when the same video datais successively input to the latch circuit, power consumption in thelatch circuit can be reduced, and low power consumption of the drivercircuit can be achieved.

<Measurements of Off-state Current of OS Transistor>

To demonstrate an “extremely low off-state current” of the OStransistor, measurement results of the off-state current of a transistorincluding a highly purified oxide semiconductor are described below.

First, in consideration of the very low off-state current of thetransistor including a highly purified oxide semiconductor, a transistorwith a channel width W of 1 m, which is sufficiently wide, was prepared,and its off-state current was measured. Results are shown in FIG. 18. InFIG. 18, the horizontal axis represents gate voltage V_(G) and thevertical axis represents drain current I_(D). In the case where thedrain voltage V_(D) is +1 V or +10 V and the gate voltage V_(G) is in arange of −5 V to −20 V, the off-state current of the transistor wasfound to be less than or equal to 1×10⁻¹² A, which is the detectionlimit. Moreover, the off-state current per unit channel width (1 μm) wasfound to be less than or equal to 1 aA (1×10⁻¹⁸ A).

Next, results obtained by more accurate measurements of the off-statecurrent of the transistor including a highly purified oxidesemiconductor are described. As described above, the off-state currentof the transistor including a highly purified oxide semiconductor isfound to be less than or equal to 1×10⁻¹² A, which is the detectionlimit of the measurement equipment. Hence, an element for evaluation ofcharacteristics was fabricated, and the results obtained by moreaccurate measurements of its off-state current (the value less than orequal to the detection limit of measurement equipment in the abovemeasurements) are described.

First, the element for evaluation of characteristics which was used incurrent measurement is described with reference to FIG. 19.

In the element for evaluation of characteristics in FIG. 19, threemeasurement systems 2300 are connected in parallel. The measurementsystem 2300 includes a capacitor 2302, a transistor 2304, a transistor2305, a transistor 2306, and a transistor 2308. As the transistor 2304,the transistor 2305, the transistor 2306, and the transistor 2308,transistors including a highly purified oxide semiconductor areemployed.

In the measurement system 2300, one of a source terminal and a drainterminal of the transistor 2304, one of terminals of the capacitor 2302,and one of a source terminal and a drain terminal of the transistor 2305are electrically connected to a power source (for supplying V₂). Theother of the source terminal and drain terminal of the transistor 2304,one of a source terminal and a drain terminal of the transistor 2308,the other of the terminals of the capacitor 2302, and a gate terminal ofthe transistor 2305 are electrically connected to one another. The otherof the source terminal and drain terminal of the transistor 2308, one ofa source terminal and a drain terminal of the transistor 2306, and agate terminal of the transistor 2306 are electrically connected to apower source (for supplying V₁). The other of the source terminal anddrain terminal of the transistor 2305 and the other of the sourceterminal and drain terminal of the transistor 2306 are electricallyconnected to each other and electrically connected to an outputterminal.

A potential V_(ext) _(—) _(b2) for controlling an on state and an offstate of the transistor 2304 is supplied to the gate terminal of thetransistor 2304. A potential V_(ext) _(—) _(b1) for controlling an onstate and an off state of the transistor 2308 is supplied to the gateterminal of the transistor 2308. A potential V_(out) is output from theoutput terminal.

Next, a method of current measurement with the use of the element forevaluation of characteristics is described.

First, an initial period in which a potential difference is applied tomeasure the off-state current is described briefly. In the initialperiod, the potential V_(ext) _(—) _(b1) for turning on the transistor2308 is input to the gate terminal of the transistor 2308. Accordingly,a potential V₁ is supplied to a node A that is electrically connected tothe other of the source terminal and drain terminal of the transistor2304 (that is, the node electrically connected to the one of the sourceterminal and the drain terminal of the transistor 2308, the other of theterminals of the capacitor 2302, and the gate terminal of the transistor2305). Here, the potential V₁ is, for example, a high potential. Thetransistor 2304 is off.

After that, a potential V_(ext) _(—) _(b1) for turning off thetransistor 2308 is input to the gate terminal of the transistor 2308 sothat the transistor 2308 is turned off. After the transistor 2308 isturned off, the potential V₁ is set low. Still, the transistor 2304 isoff. The potential V₂ is the same potential as V₁.

Consequently, the initial period ends. In a state where the initialperiod ends, a potential difference is generated between the node A andone of the source terminal and the drain terminal of the transistor2304, and also, a potential difference is generated between the node Aand the other of the source terminal and drain terminal of thetransistor 2308. Consequently, charge flows slightly through thetransistor 2304 and the transistor 2308. That is, the off-state currentis generated.

Next, a measurement period of the off-state current is brieflydescribed. In the measurement period, the potential (that is, V₂) of theone of the source terminal and the drain terminal of the transistor 2304and the potential (that is, V₁) of the other of the source terminal anddrain terminal of the transistor 2308 are kept low. The potential of thenode A is not fixed (the node A is in a floating state) in themeasurement period. Accordingly, charge flows through the transistor2304, and the amount of charge stored in the node A changes as timepasses. The potential of the node A changes depending on the change inthe amount of charge stored in the node A. That is to say, the outputpotential V_(out) of the output terminal also changes.

FIG. 20 shows details (a timing chart) of the relationship amongpotentials in the initial period in which the potential difference isgenerated and those in the subsequent measurement period.

In the initial period, first, the potential V_(extb2) is set to apotential (high potential) at which the transistor 2304 is turned on.Accordingly, the potential of the node A comes to be V₂, that is, a lowpotential (V_(SS)). Note that a low potential (V_(SS)) is notnecessarily supplied to the node A. After that, the potential V_(ext)_(—) _(b2) is set to a potential (low potential) at which the transistor2304 is turned off, whereby the transistor 2304 is turned off. Next, thepotential V_(ext) _(—) _(b1) is set to a potential (a high potential)with which the transistor 2308 is turned on. Accordingly, the potentialof the node A comes to be V₁, that is, a high potential (V_(DD)). Afterthat, the potential V_(ext) _(—) _(b1) is set to a potential at whichthe transistor 2308 is turned off. Accordingly, the node A is broughtinto a floating state and the initial period ends.

In the following measurement period, the potential V₁ and the potentialV₂ are individually set to potentials at which charge flows to or fromthe node A. Here, the potential V₁ and the potential V₂ are lowpotentials (V_(SS)). Note that at the timing of measuring the outputpotential V_(out), it is necessary to operate an output circuit;therefore, V₁ is set to a high potential (V_(DD)) temporarily in somecases. The period in which V₁ is a high potential (V_(DD)) is set shortsuch that the measurements are not affected.

When the potential difference is generated and the measurement period isstarted as described above, the amount of charge stored in the node Achanges as time passes, which causes a change in the potential of thenode A. This means that the potential of a gate terminal of thetransistor 2305 changes and accordingly, the output potential V_(out) ofthe output terminal also changes over time.

A method of calculating the off-state current on the basis of theobtained output potential V_(out) is described below.

The relationship between the potential V_(A) of the node A and theoutput potential V_(out) is obtained in advance before the off-statecurrent is calculated so that the potential V_(A) of the node A can beobtained using the output potential V_(out). With the aboverelationship, the potential V_(A) of the node A can be expressed as afunction of the output potential V_(out) by the following Equation (1).

V _(A) =F(V _(out))  (1)

Charge Q_(A) of the node A can be expressed by the following Equation(2) with the use of the potential V_(A) of the node A, capacitance C_(A)connected to the node A, and a constant (const). Here, the capacitanceC_(A) connected to the node A is the sum of the capacitance of thecapacitor 2302 and other capacitance.

Q _(A) =C _(A) V _(A)+const  (2)

Since a current I_(A) of the node A is obtained by differentiatingcharge flowing to the node A (or charge flowing from the node A) withrespect to time, the current I_(A) of the node A is expressed by thefollowing Equation (3).

$\begin{matrix}{I_{A} = {\frac{\Delta \; Q_{A}}{\Delta \; t} = \frac{{C_{A} \cdot \Delta}\; {F\left( V_{out} \right)}}{\Delta \; t}}} & (3)\end{matrix}$

In this manner, the current I_(A) of the node A can be obtained from thecapacitance C_(A) connected to the node A and the output potentialV_(out) of the output terminal.

By the above method, it is possible to measure leakage current(off-state current) which flows between a source and a drain of atransistor in an off state.

The transistor 2304, the transistor 2305, the transistor 2306, and thetransistor 2308 each of which has a channel length L of 10 μm and achannel width W of 50 μm were fabricated using a highly purified oxidesemiconductor. In the measurement systems 2300 which are arranged inparallel, the capacitances of the capacitors 2302 were set to 100 fF, 1pF, and 3 pF.

In the measurements in this embodiment, V_(DD) was set to 5 V and V_(SS)was 0 V. In the measurement period, V_(out) was measured while thepotential V₁ was basically set to Vss and changed to V_(DD) for 100 msecat intervals of 10 sec to 300 sec. At which was used in calculation ofcurrent I which flows through the element was about 30000 sec.

FIG. 21 shows the relationship between the output potential V_(out) andelapsed time Time in the current measurements. As is seen in FIG. 21,the output potential V_(out) changes over time.

The off-state current at room temperature (25° C.) calculated in theabove current measurements is shown in FIG. 22. Note that FIG. 22 showsthe relationship between source-drain voltage V and off-state current I.FIG. 22 demonstrates that the off-state current is approximately 40zA/μm under the condition where the source-drain voltage is 4 V. Inaddition, the off-state current is less than or equal to 10 zA/μm underthe condition where the source-drain voltage is 3.1 V. Note that 1 zArepresents 1×10⁻²¹ A.

Further, the off-state current at the time when the temperature is 85°C. calculated in the above current measurements is shown in FIG. 23.FIG. 23 shows the relationship between the source-drain voltage V andthe off-state current I at the time when the temperature is 85° C. FIG.23 demonstrates that the off-state current is less than or equal to 100zA/μm when the source-drain voltage is 3.1 V.

The above results also demonstrate an extremely low off-state current ofthe transistor including an oxide semiconductor film as an active layer.

<Method of Manufacturing Driver Circuit>

Next, an example of a manufacturing process of the signal processingunit 100 illustrated in FIGS. 1A to 1C is described with reference toFIGS. 2A to 2D, FIGS. 3A to 3D, FIGS. 4A to 4D, FIGS. 5A and 5B, FIGS.6A and 6B, FIG. 7, and FIGS. 8A to 8C. Note that hereinbelow, a methodof forming the circuit layer 110 in the signal processing unit 100 isdescribed and then a method in which the circuit layer is provided overa flexible substrate to fabricate the signal processing unit 100 isdescribed.

<Method of Forming Circuit Layer 110>

First, an example of a formation process of the circuit layer 110illustrated in FIGS. 1A to 1C is described using FIGS. 2A to 2D, FIGS.3A to 3D, FIGS. 4A to 4D, FIGS. 5A and 5B, FIGS. 6A and 6B, and FIG. 7.Note that although a variety of elements (e.g., a transistor) areactually formed in the first layer 111 and the second layer 112 in thecircuit layer 110, for easier understanding of the formation process inthis embodiment, a method of forming the transistor including a singlecrystal semiconductor film as an active layer is mainly described as amethod of forming the first layer and a method of manufacturing the OStransistor is mainly described as a method of manufacturing the secondlayer. It is needless to say that elements other than such transistorsare present in the first layer and the second layer.

<Method of Manufacturing First Layer 111>

First, a semiconductor substrate 300 is prepared and an insulating film302 is formed on a surface of the substrate (see FIG. 2A).

As the semiconductor substrate 300, it is possible to use a substrateformed of a Group 14 element, such as a single crystal siliconsubstrate, a single crystal germanium substrate, or a single crystalsilicon germanium substrate, for example. Further, a compoundsemiconductor substrate using gallium nitride, gallium arsenide, indiumphosphide, or the like can be used. A commercially-available siliconsubstrate is typically a circular substrate having a size of 5 inches(125 mm) in diameter, 6 inches (150 mm) in diameter, 8 inches (100 mm)in diameter, 12 inches (300 mm) in diameter, and 16 inches (700 mm) indiameter. The shape of the semiconductor substrate 300 is not limited toa circular shape, and the semiconductor substrate 300 may be a substratethat is processed into a rectangular shape, a square shape, or the like,for example. The semiconductor substrate 300 can be formed by theCzochralski (CZ) method or the floating zone (FZ) method.

In this embodiment, a single crystal silicon substrate is used as thesemiconductor substrate 300.

The insulating film 302 may be formed with a single layer of a siliconoxide film, a silicon oxynitride film, or the like, or a stacked layerof any of these films, for example. As a method of forming the film, athermal oxidation method, a CVD method, a sputtering method, or the likecan be used. When the insulating film 302 is formed by a CVD method, asilicon oxide film is preferably formed using an organosilane such astetraethoxysilane (abbreviation: TEOS, chemical equation: Si(OC₂H₅)₄) inorder to obtain favorable bonding to another substrate performed later.

Note that in the case where the insulating film 302 is formed by thermaloxidation treatment, the thermal oxidation treatment is preferablyperformed in an oxidizing atmosphere to which halogen is added. Forexample, by performing a thermal oxidation treatment on thesemiconductor substrate 300 in an oxidizing atmosphere to which chlorine(Cl) is added, the insulating film 302 can be formed through chlorineoxidation. In this case, the insulating film 302 is a film containingchlorine atoms. Such chlorine oxidation prevents impurities such as Nafrom entering the semiconductor substrate 300 from the first temporaryfixing substrate 314 which is bonded to the semiconductor substrate 300in a later process, by which contamination of the semiconductorsubstrate 300 can be prevented. Note that the halogen atoms contained inthe insulating film 302 are not limited to chlorine atoms. Theinsulating film 302 may contain fluorine atoms.

It is preferable that the surface of the semiconductor substrate 300 becleaned with hydrochloric acid/hydrogen peroxide mixture (HPM), asulfuric acid/hydrogen peroxide mixture (SPM), an ammonium hydrogenperoxide mixture (APM), diluted hydrofluoric acid (DHF), a mixedsolution of hydrofluoric acid, hydrogen peroxide water, ozone water, andpure water (FPM), or the like before forming the insulating film 302.

Then, an ion irradiation treatment 304 is performed from a surface ofthe semiconductor substrate 300, so that an embrittled region 306 isformed in the semiconductor substrate 300 (see FIG. 2B).

Hydrogen ions can be used for ion species for the irradiation. Thehydrogen ions refer to a mixture of one or more kinds of ions selectedfrom H⁺, H₂ ⁺, and H₃ ⁺.

When irradiation with accelerated hydrogen ions is performed, theproportion of H₃ ⁺ is preferably set high. Specifically, it ispreferable that the proportion of H₃ ⁺ in the total amount of H⁺, H₂ ⁺,and H₃ ⁺ is set 50% or higher (more preferably, 80% or higher). With ahigh proportion of H₃ ⁺, the efficiency of ion irradiation can beimproved. Further, rare gas ions can be used as well as hydrogen ions.Specifically, He ions, Ne ions, Ar ions, Kr ions, and Xe ions can beused.

The depth at which the embrittled region 306 is formed can be controlledby the kinetic energy, mass, charge, or incidence angle of the ions forthe irradiation, or the like. The embrittled region 306 is formed atapproximately the same depth as the average penetration depth of theions. Thus, by adjusting the kind of ion species for the irradiation andthe condition of the irradiation, the thickness of a semiconductor thinfilm 316 to be separated from the semiconductor substrate 300 in a laterprocess can be adjusted.

The thickness of the semiconductor thin film 316 is not particularlylimited, but is preferably greater than or equal to 1 nm and less thanor equal to 200 nm, more preferably greater than or equal to 3 nm andless than or equal to 100 nm because in the case where the separatedsemiconductor thin film 316 having too large thickness is used forforming a high performance semiconductor integrated circuit, thetransistor might cause an increase in S value or might be normally on,for example. Accordingly, the average penetration depth of the ions forthe irradiation is preferably adjusted so that the depth at which theembrittled region 306 is formed in the semiconductor substrate 300 isgreater than or equal to 1 nm and less than or equal to 200 nm, morepreferably greater than or equal to 3 nm and less than or equal to 100nm.

The ion irradiation treatment 304 can be performed with an ion dopingapparatus or an ion implantation apparatus. With an ion implantationapparatus, ion species in plasma are subjected to mass separation, and asemiconductor substrate can be irradiated with only ion species having acertain mass, so that entry of impurities which affect thecharacteristics of a transistor can be reduced. Thus, an ionimplantation apparatus is especially preferable.

However, even when the ion irradiation treatment 304 is performed withthe use of an ion doping apparatus, substances which affect thecharacteristics of a transistor (e.g., heavy metal) can be trapped byperforming the ion irradiation treatment 304 through the insulating film302.

Next, a separation layer 310 is formed over the insulating film 302close to the side on which the embrittled region 306 is formed (see FIG.2C). The separation layer 310 may be a single layer or stacked layers.Note that the separation layer 310 illustrated in FIG. 2C has a stackedlayer structure including a first separation layer 307 and a secondseparation layer 308.

The separation layer is formed to have a single-layer structure or astacked layer structure by using an element such as tungsten (W),molybdenum (Mo), titanium (Ti), tantalum (Ta), niobium (Nb), nickel(Ni), cobalt (Co), zirconium (Zr), zinc (Zn), ruthenium (Ru), rhodium(Rh), palladium (Pd), osmium (Os), iridium (Ir), or silicon (Si); or analloy or a compound containing any of the elements as its main componentby a sputtering method, a plasma CVD method, a coating method, aprinting method, or the like.

When the separation layer 310 has a single-layer structure, a tungstenfilm, a molybdenum film, or a film containing a mixture of tungsten andmolybdenum is preferably formed. A film containing an oxide of tungstenor a film containing an oxynitride of tungsten may be formed. A filmcontaining an oxide of molybdenum or a film containing an oxynitride ofmolybdenum may be formed. A film containing an oxide or oxynitride of amixture of tungsten and molybdenum may be formed. Note that the mixtureof tungsten and molybdenum corresponds to an alloy of tungsten andmolybdenum.

When the separation layer 310 has a stacked layer structure, preferably,a metal film is formed as the first separation layer 307 and a metaloxide film is formed as the second separation layer 308. Typically, afilm including tungsten, a film including molybdenum, or a filmincluding a mixture of tungsten and molybdenum is formed as the firstseparation layer 307 and an oxide film or an oxynitride film of a filmincluding tungsten, a film including molybdenum, or a film including amixture of tungsten and molybdenum is formed as the second separationlayer 308.

The second separation layer 308 may be formed as follows: the firstseparation layer 307 is formed as a metal film, and an oxide film 309 isformed over the first separation layer 307, by which a surface of thefirst separation layer 307 is oxidized to result in the secondseparation layer 308 as a metal oxide layer. In this case, theseparation layer 310 has a stacked layer structure of the firstseparation layer 307, the second separation layer 308, and the oxidefilm 309 as illustrated in FIG. 2D. For example, in the case where atungsten film is formed as the first separation layer 307 and, over thefilm, a silicon oxide film is formed as the oxide film 309, the tungstenfilm is oxidized in the vicinity of an interface with the silicon oxidefilm and a film including tungsten oxide is formed as the secondseparation layer 308.

The second separation layer 308 which is a metal oxide film may beformed by subjecting a surface of the first separation layer 307 tothermal oxidation treatment, oxygen plasma treatment, N₂O plasmatreatment, UV ozone treatment, treatment with a solution having strongoxidizing power such as ozone water, or the like.

Alternatively, as the separation layer 310, a metal film and a metaloxynitride film may be formed as the first separation layer 307 and thesecond separation layer 308, respectively. Typically, a film includingtungsten is formed as the first separation layer 307 and a tungstenoxynitride film is formed as the second separation layer 308. The metaloxynitride film is more easily separated with a higher proportion ofoxygen, while the metal oxynitride film has more difficulty in beingseparated with a higher proportion of nitrogen. Hence, the proportion ofoxygen and nitrogen in the film can be determined as appropriateconsidering a stress (e.g., stress application or heating) applied tothe separation layer 310 in a later process.

In this embodiment, a 30-nm-thick tungsten film formed by a sputteringmethod is used as the first separation layer 307 and, over the film, a100-nm-thick silicon oxide film is formed by a sputtering method andused as the oxide film 309. Consequently, as illustrated in FIG. 2D, theseparation layer 310 has a structure in which tungsten oxide serving asthe second separation layer 308 is formed in the vicinity of a surfaceof the first separation layer 307 which faces the oxide film 309. Notethat in FIG. 2D and the subsequent figures, a layer including the firstseparation layer 307, the second separation layer 308, and the oxidefilm 309 is referred to as the separation layer 310.

Note that a bonding layer may be formed over the separation layer 310 inorder to increase the adhesion with a first temporary fixing substrate314 to be bonded to the separation layer 310 in a later process. Thebonding layer preferably has a highly flat surface and uses a materialcapable of forming a hydrophilic surface. As the bonding layer, asilicon oxide film is suitable. In particular, a silicon oxide filmformed by a chemical vapor deposition method using a silane-based gassuch as a silane gas, a disilane gas, a trisilane gas, or anorganosilane gas, is preferable. When the silane gas is used, a gaswhich is mixed with nitrogen dioxide or dinitrogen monoxide ispreferably used. Examples of the organosilane gas includesilicon-containing compounds, such as tetraethyl orthosilicate (TEOS,chemical equation: Si(OC₂H₅)₄), trimethylsilane (TMS, chemical equation:(CH₃)₃SiH), tetramethylcyclotetrasiloxane (TMCTS),octamethylcyclotetrasiloxane (OMCTS), hexamethyldisilazane (HMDS),triethoxysilane (chemical equation: SiH(OC₂H₅)₃), ortrisdimethylaminosilane (chemical equation: SiH(N(CH₃)₂)₃). As achemical vapor deposition method, plasma CVD, thermal CVD, or photo CVDmay be used.

Regarding the above-given term “hydrophilic”, when a liquid (e.g.,water) dropped onto a surface gives a contact angle between the droppedliquid and the surface of less than or equal to 30°, the surface is ahydrophilic surface. The contact angle is preferably less than 10° (inthis case, the term superhydrophilic or the like is used).

The bonding layer having a highly flat surface with hydrophilicity isprovided to a thickness of from 5 nm to 500 nm. The bonding layer havingsuch a thickness can reduce the influence caused by the rough surface ofthe separation layer. In addition, it is possible to reduce distortionbetween the first temporary fixing substrate to be bonded later and thesubstrate.

After the bonding layer is formed, the surface of the bonding layer ispreferably subjected to planarization treatment. As the planarizationtreatment, reverse sputtering treatment, dry etching treatment, chemicalmechanical polishing (CMP) treatment, or the like can be used. The“reverse sputtering treatment” here refers to a method in which, withoutapplication of a voltage to a target side, RF power is applied to asubstrate side in an argon atmosphere to generate plasma in the vicinityof the substrate and modify and planarize a surface. Note that insteadof an argon atmosphere, a nitrogen atmosphere, a helium atmosphere, orthe like may be used. Alternatively, an argon atmosphere to whichoxygen, nitrous oxide, or the like is added may be used. Alternatively,an argon atmosphere to which chlorine, carbon tetrafluoride, or the likeis added may be used.

Next, the first temporary fixing substrate 314 is prepared and bonded tothe side of the semiconductor substrate 300 where the separation layer310 is formed (see FIG. 3A).

As the first temporary fixing substrate 314, a polycrystallinesemiconductor substrate or a single crystal semiconductor substrate canbe used. As the polycrystalline semiconductor substrate or the singlecrystal semiconductor substrate, for example, a semiconductor substratethat is formed of an element which belongs to the Group 14, such as apolycrystalline or single crystal silicon substrate, a polycrystallineor single crystal germanium substrate, a polycrystalline or singlecrystal silicon germanium substrate, or a polycrystalline or singlecrystal silicon carbide substrate, a polycrystalline or single crystalcompound semiconductor substrate using gallium arsenide, indiumphosphide, or the like can be given. Typical examples of siliconsubstrates are circular silicon substrates which are 5 inches (125 mm)in diameter, 6 inches (150 mm) in diameter, 8 inches (200 mm) indiameter, and 12 inches (300 mm) in diameter. Note that the shape is notlimited to the circular shape, and a substrate processed into arectangular shape or the like can also be used. In this embodiment, asilicon substrate processed into a square shape is used as the firsttemporary fixing substrate 314.

As the first temporary fixing substrate 314, a substrate formed of aninsulator can also be used. For example, a variety of glass substratesfor electronic industry such as aluminosilicate glass,aluminoborosilicate glass, and barium borosilicate glass, a naturalquartz substrate, a synthetic quartz substrate, or the like can be used.A metal substrate can also be used as the first temporary fixingsubstrate 314.

Because the circuit layer 110 functioning as the driver circuit isformed over the first temporary fixing substrate 314 in a later process,a substrate that can be tolerant to a patterning process atsubmicrometer levels is preferably used as the first temporary fixingsubstrate 314. Among the above-mentioned materials for the firsttemporary fixing substrate, for example, a single crystal siliconsubstrate is more flat and less undulated over a wide range than asubstrate formed of an insulator or a metal substrate.

Note that a surface of the first temporary fixing substrate 314 ispreferably cleaned in advance by the cleaning methods similar to thosefor the semiconductor substrate 300. Such cleaning treatment can improvethe flatness of the surface of the first temporary fixing substrate 314and remove particles or organic materials on the surface of the firsttemporary fixing substrate 314. This can increase adhesion between theseparation layer 310 and the first temporary fixing substrate 314(adhesion between the bonding layer and the first temporary fixingsubstrate 314 when the bonding layer is formed over the separation layer310) and prevent a failure in transfer (e.g., the semiconductor thinfilm cannot be separated from the semiconductor substrate 300 or theseparated semiconductor thin film does not adhere to the first temporaryfixing substrate 314).

Before the semiconductor substrate 300 and the first temporary fixingsubstrate 314 are bonded to each other, surfaces to be bonded arepreferably subjected to cleaning treatment. As the cleaning treatment,wet treatment, dry treatment, or a combination of wet treatment and drytreatment can be used. Alternatively, wet treatment may be used incombination with different wet treatment or dry treatment may be used incombination with different dry treatment.

The intensity of bonding between the separation layer 310 and the firsttemporary fixing substrate 314 (between the bonding layer and the firsttemporary fixing substrate 314 when the bonding layer is formed over theseparation layer 310) largely depends on surface attractive force(so-called van der Waals force); therefore, the intensity of the bondingcan be increased by making the surfaces to be bonded hydrophilic byforming hydroxyl groups thereon. As hydrophilicity treatment, forexample, any one of or a combination of two or more of oxygen plasmatreatment, N₂ plasma treatment, Ar plasma treatment, ozone treatment, UVozone treatment, and ozone water treatment can be performed.

Before and after the bonding, heat treatment may be performed in orderto increase the intensity of bonding between the separation layer 310(the bonding layer when the bonding layer is formed over the separationlayer 310) and the first temporary fixing substrate 314. In the heattreatment performed before the bonding, the semiconductor substrate 300and the first temporary fixing substrate 314 are bonded to each otherwhile the semiconductor substrate 300 is heated at a temperature greaterthan or equal to 50° C. and less than 150° C. In the heat treatmentperformed after the bonding, the semiconductor substrate 300 may beheated at a temperature at which separation (also referred to as acleavage phenomenon) does not occur at the embrittled region 306; forexample, the semiconductor substrate 300 is heated at a temperaturegreater than or equal to 100° C. and less than 400° C. The above heattreatment can be performed using as a resistance heating furnace, arapid thermal annealing (RTA) apparatus, a microwave heating apparatus,or the like.

Next, the semiconductor substrate 300 to which the first temporaryfixing substrate 314 is bonded is subjected to heat treatment in orderto perform the transfer process. The heat treatment leads to a volumechange of the embrittled region 306 (e.g., voltage change of the ions(either or both of hydrogen ions or rare gas ions) or molecules of theions in the embrittled region 306, which are added to the semiconductorsubstrate 300 by the ion irradiation treatment 304), which results incleavage in the embrittled region 306. Note that the heat treatment ispreferably performed at a temperature from 400° C. to the uppertemperature limit of the substrate whose upper temperature limit islower (the semiconductor substrate 300 or the first temporary fixingsubstrate 314), for example, at a temperature from 400° C. to 600° C.After that, the semiconductor substrate 300 is separated from the firsttemporary fixing substrate 314, so that the semiconductor thin film 316,which is separated from the semiconductor substrate 300 at theembrittled region 306 as the cleavage plane, is transferred onto thefirst temporary fixing substrate 314 through the insulating film 302 andthe separation layer 310 (see FIG. 3B).

Note that a method different from the method in this embodiment isdescribed in Embodiment 2.

The above heat treatment for separating the semiconductor thin film 316is not necessarily needed. For example, the following method may beused: a stress is applied to the embrittled region 306 (e.g., by cuttingthe embrittled region 306 with a thin edged tool in the directionparallel to the embrittled region 306 is formed) so that thesemiconductor thin film 316 is mechanically separated from thesemiconductor substrate 300. The above mechanical separation methodperformed at the same time as the heat treatment allows thesemiconductor thin film 316 to be separated from the semiconductorsubstrate 300 even at a temperature less than 400° C.

The semiconductor thin film 316 provided over the first temporary fixingsubstrate 314 by the above process may include part of the embrittledregion 306 in its surface. The embrittled region 306 may include moredefect regions than the semiconductor substrate 300. In addition, sincethe surface of the semiconductor thin film 316 corresponds to theembrittled region 306 which is subjected to ion irradiation, the surfaceflatness may be low. For these reasons, the semiconductor thin film 316separated from the semiconductor substrate 300 is preferably subjectedto treatment for removing the embrittled region 306 and planarizing thesurface. As a method of the treatment, chemical mechanical polishing(CMP) treatment, a dry etching method, reverse sputtering treatment, orthe like can be used. This leads to the structure illustrated in FIG.3C, in which the embrittled region 306 is removed. Alternatively, thesurface of the semiconductor thin film 316 may be planarized by beingirradiated with laser light so that the surface of the semiconductorthin film 316 is melted.

Next, a semiconductor element is formed using the semiconductor thinfilm 316. Note that in this embodiment, as an example, a method ofmanufacturing an n-channel transistor and a p-channel transistor eachusing the semiconductor thin film 316 as an active layer is described.It is needless to say that the semiconductor element using thesemiconductor thin film 316 is not limited to the pair of n-channeltransistor and p-channel transistor.

First, a resist mask is formed over the semiconductor thin film 316 by aphotolithography method, a printing method, an inkjet method, or thelike, and part of the semiconductor thin film 316 and part of theinsulating film 302 are selectively removed using the resist mask,whereby an island-shaped semiconductor film 400 a and an island-shapedsemiconductor film 400 b are formed, and then the resist mask isremoved. Then, if necessary, a minute amount of an impurity is added tothe island-shaped semiconductor films to control the threshold voltage(so-called channel doping). In order to obtain the desired thresholdvoltage, an impurity imparting n-type or p-type conductivity (such asphosphorus or boron) is added by an ion doping method or the like. Notethat in the description in this embodiment, the semiconductor film 400 ais used for the n-channel transistor and the semiconductor film 400 b isused for the p-channel transistor.

An element that belongs to the Group 15, such as phosphorus (P) orarsenic (As), can be used as the impurity element imparting n-typeconductivity, and the element can be added to the semiconductor film 400a by an ion implantation method, an ion doping method, a plasmaimmersion ion implantation method, or the like. An element that belongsto the Group 13, such as boron (B), aluminum (Al), or gallium (Ga), canbe used as the impurity element imparting p-type conductivity, and theelement can be added to the semiconductor film 400 b by any of the abovemethods. Note that a suitable condition can be selected as appropriateto determine the amount of the impurity elements to be added consideringnecessary properties of the transistors.

Next, an insulating film 401 covering the semiconductor film 400 a andthe semiconductor film 400 b is formed (see FIG. 4A). Since thesemiconductor film 400 a and the semiconductor film 400 b in thisembodiment are highly flat, even when the insulating film 401 is a thinfilm, a defect due to the unevenness of the semiconductor films (i.e.,the roughness of a surface or a side of the semiconductor films) is notgenerated. Thus, a property defect due to insufficient coverage with theinsulating film 401 can be prevented, which enables high-yieldmanufacture of a highly reliable semiconductor element. Note that as theinsulating film 401 is thinner, the transistor can be driven at higherspeed with lower voltage.

The insulating film 401 can be formed by a physical vapor deposition(PVD) method such as a vacuum deposition method or a sputtering method,or a chemical vapor deposition (CVD) method such as a plasma CVD methodto have a single-layer structure or a stacked layer structure of asilicon oxide film, a silicon oxynitride film, a silicon nitride film, asilicon nitride oxide film, an aluminum oxide film, an aluminumoxynitride film, an aluminum nitride oxide film, an aluminum nitridefilm, or the like. Note that in this specification, a “siliconoxynitride film” refers to a film that includes more oxygen thannitrogen, and a “silicon nitride oxide film” refers to a film thatincludes more nitrogen than oxygen. In this embodiment, the insulatingfilm 401 is formed by plasma treatment because a film formed by plasmatreatment is dense and has high withstand voltage and excellentreliability.

Alternatively, the insulating film 401 may be formed by oxidation ornitridation of the surfaces of the semiconductor films 400 a and 400 b.For example, the oxidation or nitridation is carried out by usingnitrous oxide (N₂O) diluted with Ar by 1 time to 3 times (flow rateratio) while generating plasma by application of a microwave (2.45 GHz)power of 3 kW to 5 kW to an electrode at a pressure of 10 Pa to 30 Pa.By this treatment, an insulating film having a thickness of 1 nm to 30nm (preferably 2 nm to 20 nm) is formed. Further, this process may befollowed by the formation of a silicon oxynitride film with a vapordeposition method which is conducted in nitrous oxide (N₂O) and silane(SiH₄) under application of a microwave (2.45 GHz) power of 3 kW to 5 kWto an electrode at a pressure of 10 Pa to 30 Pa. With a combination ofsolid-phase reaction and vapor deposition, the insulating film 401 isable to have low interface state density and high withstand voltage canbe formed. Note that the insulating film 401 functions as a gateinsulating film.

As a material for the insulating film 401, a high dielectric constantmaterial (also referred to as a high-k material, for example) such aszirconium dioxide, hafnium oxide, titanium dioxide, or tantalumpentoxide may be used. The use of a high dielectric constant materialfor the insulating film 401 enables a reduction in the gate leakagecurrent of a gate insulating film 402 a and a gate insulating film 402 bformed in a later process.

In this embodiment, a 15-nm-thick silicon oxide film is formed usingplasma generated by the above microwave and used as the insulating film401.

Next, after a conductive film is formed over the insulating film 401, aresist mask is formed over the conductive film by a photolithographymethod, a printing method, an inkjet method, or the like. Part of theconductive film and the insulating film 401 is selectively removed withthe resist mask, whereby a gate electrode 404 a, a gate electrode 404 b,the gate insulating film 402 a, and the gate insulating film 402 b areformed, and then the resist mask is removed (see FIG. 4B).

The conductive film serving as the gate electrode 404 a and the gateelectrode 404 b may be formed of a film including an element selectedfrom tantalum (Ta), tungsten (W), titanium (Ti), molybdenum (Mo),aluminum (Al), copper (Cu), chromium (Cr), and neodymium (Nd), or analloy material such as AgPdCu or compound material mainly containing anyof the elements by a physical vapor deposition (PVD) method such as avacuum evaporation method or a sputtering method or a chemical vapordeposition (CVD) method such as a plasma CVD method. Further, as theconductive film serving as the gate electrodes, a semiconductor filmtypified by a polycrystalline silicon film doped with an impurityelement such as phosphorus may be used.

In this embodiment, as the conductive film serving as the gate electrode404 a and the gate electrode 404 b, a stacked layer film of a30-nm-thick tantalum nitride film and a 170-nm-thick tungsten film isformed.

Next, a mask covering the semiconductor film 400 b is formed, thesemiconductor film 400 a is subjected to treatment for adding animpurity element imparting n-type conductivity, and then the mask isremoved. Consequently, portions of the semiconductor film 400 a which donot overlap with the gate electrode 404 a serve as an n-type impurityregion 410 a and an n-type impurity region 410 b, and a portion of thesemiconductor film 400 a which overlaps with the gate electrode 404 afunctions as a channel formation region 410 c. After that, a maskcovering the semiconductor film 400 a is formed, the semiconductor film400 b is subjected to treatment for adding an impurity element impartingp-type conductivity, and the mask is removed. Consequently, portions ofthe semiconductor film 400 b which do not overlap with the gateelectrode 404 b serve as a p-type impurity region 416 a and a p-typeimpurity region 416 b, and a portion of the semiconductor film 400 bwhich overlaps with the gate electrode 404 b functions as a channelformation region 416 c (see FIG. 4C). Note that there is no limitationon the order of addition of the impurity element imparting n-typeconductivity and addition of the impurity element imparting p-typeconductivity.

Any of the elements given in the description of channel doping can beused as the impurity element imparting n-type conductivity and theimpurity element imparting p-type conductivity, and a suitable conditioncan be selected as appropriate to determine the amount of the impurityelements to be added considering necessary properties of the transistor.

By the above-described process, an n-channel transistor 438 and ap-channel transistor 440 using the semiconductor thin film 316 as anactive layer are formed (see FIG. 4C). The structure of the transistordescribed in this embodiment is merely an example and does not limit thepresent invention. A semiconductor element other than a transistor maybe included. In addition to a semiconductor element, an element usingvarious films used for forming the semiconductor element (e.g., acapacitor including an insulating film, a conductive film, or the like)may be formed.

Next, an interlayer insulating layer 450 which is a single layer or hasa stacked layer structure is formed to cover the n-channel transistor438 and the p-channel transistor 440 described above. Thus, the firstlayer 111 having the n-channel transistor and the p-channel transistorincluding the semiconductor thin film 316 as an active layer is formed(see FIG. 4D).

The interlayer insulating layer 450 may be formed in such a manner thatan organic material having an insulating property is applied by a spincoating method, a printing method, a dispensing method, an ink-jetmethod, or the like, and cure treatment (e.g., heat treatment or lightirradiation treatment) is performed depending on the applied material.As the organic material having an insulating property, for example, anorganic resin such as an acrylic resin, a polyimide resin, a polyamideresin, a polyamide-imide resin, or an epoxy resin can be used. Inaddition to organic materials, an inorganic material can be used. It isalso possible to use a low-dielectric constant material (low-kmaterial), a siloxane-based resin, phosphosilicate glass (PSG),borophosphosilicate glass (BPSG), or the like.

In this embodiment, a 500-nm-thick silicon oxynitride film is formed andused as the interlayer insulating layer 450.

In order that the second layer including the OS transistor be formed ina later process, a wiring layer 455, which is electrically connected tothe n-channel transistor 438 and the p-channel transistor 440 through anopening provided in the interlayer insulating layer 450, is formed overthe first layer 111 (see FIG. 4D).

The wiring layer 455 can be formed using a method and a material similarto those of the gate electrode 404 a. The gate electrodes 404 a and 404b are not directly connected to the wiring layer 455 in FIG. 4D, butthis structure does not limit the present invention.

In this embodiment, a 150-nm-thick tungsten film is formed and used asthe wiring layer 455.

After the wiring layer 455 is formed, an interlayer insulating layer 462which is a single layer or has a stacked layer structure is furtherformed over the interlayer insulating layer 450 and the wiring layer 455(see FIG. 5A). The interlayer insulating layer is preferably planarized.With this layer, when another circuit layer (the second layer 112 inthis embodiment) is formed over the first layer 111, it is possible toprevent, for example, disconnection of a film formed over the firstlayer 111, resulting in fewer defects in the circuit layer 110 which iscaused by roughness generated due to a component of the first layer 111(e.g., the n-channel transistor 438 or the p-channel transistor 440).

For the interlayer insulating layer 462, the method and material of theinsulating film 302 or the interlayer insulating layer 450 can be used.As the planarization treatment, a method similar to that for the bondinglayer can be used.

In this embodiment, as the interlayer insulating layer 462, a400-nm-thick silicon oxide film formed with a TEOS and subjected to CMPtreatment as the planarization treatment is used.

Over the interlayer insulating layer 462, a wiring layer 464electrically connected to one or more components formed in the firstlayer 111 (the n-channel transistor 438 and the p-channel transistor 440in this embodiment) and an interlayer insulating layer 466 which is asingle layer or has a stacked layer structure are formed.

The wiring layer 464 is used to electrically connect the componentsformed in the first layer 111 in the above process (the n-channeltransistor 438 and the p-channel transistor 440 in this embodiment) tocomponents formed in the second layer formed in a later process.

Owing to electrical connection between the n-channel transistor 438 andthe p-channel transistor 440 through the wiring layer 464 as in FIG. 5A,for example, an analog switch (or part of an analog switch) capable ofbeing driven at high speed in which the active layer is formed with thesingle crystal semiconductor film can be formed (see FIG. 6B).

In the step of forming the wiring layer 464, a back gate electrode ofthe OS transistor formed in a later process may be formed. This ispreferable because application of a voltage to the back gate electrodesurely enables the transistor to be a normally-off transistor.

For the wiring layer 464, the method and material of the gate electrodes404 a and 404 b can be used. Note that in this embodiment, like thewiring layer 455, a 150-nm-thick tungsten film is formed and used.

Preferably, the interlayer insulating layer 466 is formed using themethod and material of the insulating film 302 and subjected toplanarization treatment so that roughness generated in the formation ofthe wiring layer 464 is reduced. In this embodiment, a silicon nitrideoxide film is formed over the interlayer insulating layer 462 and thewiring layer 464 and subjected to planarization treatment by CMP, foruse as the interlayer insulating layer 466.

An interlayer insulating layer 468 is preferably formed between thesecond layer 112 formed in a later process and the above-described firstlayer 111 (over the wiring layer 464 and the interlayer insulating layer466 in this embodiment).

The interlayer insulating layer 468 is provided in order to suppressdispersion of impurities, which adversely affect electriccharacteristics of the transistors, between the first layer 111 and thesecond layer 112.

With respect to the OS transistor formed in a later process, if a largeamount of hydrogen is contained in the oxide semiconductor film,hydrogen is bonded to the oxide semiconductor and partly becomes a donorto generate electrons serving as carriers. This might cause problemssuch as a shift of the threshold voltage of the transistors in thenegative direction, greater variations in initial characteristics of thetransistors, an increase in the channel length dependence of electricalcharacteristics of the transistors, and larger degradation of theelectric characteristics in BT stress tests. Therefore the hydrogenconcentration in the oxide semiconductor film is preferably less than7.2×10²⁰ atoms/cm³. Alternatively, the hydrogen concentration in theoxide semiconductor film is preferably less than or equal to 5×10¹⁹atoms/cm³.

Thus, it is encouraged to suppress dispersion of hydrogen from the firstlayer 111 including the transistor including a single crystalsemiconductor film (a single crystal silicon film in this embodiment) asan active layer into the second layer 112 including the OS transistor.In this embodiment, a 50-nm-thick aluminum oxide film having such afunction is formed by a sputtering method and used as the interlayerinsulating layer 468. The aluminum oxide film preferably has a highdensity (film density greater than or equal to 3.2 g/cm³, preferablygreater than or equal to 3.6 g/cm³).

In this embodiment, a 50-nm-thick aluminum oxide film is formed by asputtering method and used as the interlayer insulating layer 468.

<Method of Forming Second Layer 112>

Next, a method of forming the second layer 112 including thesemiconductor element using the oxide semiconductor thin film over thefirst layer 111 is described.

First, an insulating film 700 is formed over the interlayer insulatinglayer 468 and an island-shaped oxide semiconductor film 702 is formedover the insulating film 700 (see FIG. 5B). The insulating film 700 canbe formed using a method and a material similar to those of theinsulating film 302.

As the insulating film 700 (an insulating film on the outmost surfacewhen the insulating film 700 has a stacked layer structure), a film fromwhich oxygen can be released by heat treatment (hereinafter referred toas an oxygen supply film) is preferably formed. The reason is describedbelow.

The oxide semiconductor film is formed over the insulating film 700.When an oxygen vacancy exists in the channel formation region of the OStransistors, charge is generated due to the oxygen vacancy in somecases. Part of the oxygen vacancy in an oxide semiconductor film servesas a donor to release an electron which is a carrier. As a result, thethreshold voltage of the transistor shifts in the negative direction.

When the insulating film 700 (an insulating film on the outmost surfacewhen the insulating film 700 has a stacked layer structure) functions asan oxygen supply film, part of oxygen in the oxygen supply film can bereleased by heat treatment. Accordingly, after the oxide semiconductorfilm is formed, the oxygen supply film is heated to supply oxygen to theoxide semiconductor film, so that the oxygen vacancy in the oxidesemiconductor film can be filled. This can suppress a shift of thethreshold voltage of the OS transistor in the negative direction. Inparticular, the oxygen supply film preferably contains oxygen whichexceeds the stoichiometric composition. For example, in the case wheresilicon oxide is used for the oxygen supply film, a film of siliconoxide represented by SiO_(2+α) (α>0) is preferably used. Note that it isacceptable as long as a region containing a larger amount of oxygen thanthe stoichiometric composition (hereinafter referred to as “anoxygen-excessive region” in some cases) exists in at least part of theoxygen supply film.

Note that the “film from which oxygen can be released by heat treatment”described above refers to a film whose amount of oxygen released whenconverted into oxygen atoms in thermal desorption spectroscopy (TDS)analysis is greater than or equal to 1.0×10¹⁹ atoms/cm³, greater than orequal to 3.0×10¹⁹ atoms/cm³, greater than or equal to 1.0×70²⁰atoms/cm³, or greater than or equal to 3.0×70²⁰ atoms/cm³.

Here, a method in which the amount of released oxygen is measured bybeing converted into oxygen atoms using the TDS analysis is described.

The amount of released gas in the TDS analysis is proportional to theintegral value of a spectrum. Thus, the amount of released gas can becalculated from the ratio between the integral value of a measuredspectrum and the reference value of a standard sample. The referencevalue of a standard sample refers to the ratio of the density of apredetermined atom contained in a sample to the integral value of aspectrum.

For example, the number of released oxygen molecules (N_(O2)) from aninsulating film can be obtained according to Equation (4) with the TDSanalysis of a silicon wafer containing hydrogen at a predetermineddensity which is the standard sample and the TDS analysis of theinsulating film. Here, all spectra having a mass-to-charge ratio (M/z)of 32 which are obtained by the TDS analysis are assumed to originatefrom an oxygen molecule. Note that CH₃OH, where M/z=32, is not takeninto consideration on the assumption that it is unlikely to be present.Further, an oxygen molecule including an isotope oxygen atom of M/z=17or M/z=18 is not taken into consideration either because the proportionof such a molecule in the natural world is minimal.

$\begin{matrix}{N_{O_{2}} = {\frac{N_{H_{2}}}{S_{H_{2}}} \times S_{O_{2}} \times \alpha}} & (4)\end{matrix}$

The value N_(H2) is obtained by conversion of the number of hydrogenmolecules desorbed from the standard sample into densities. The integralvalue of a spectrum when the standard sample is subjected to the TDSanalysis is denoted by S. Here, the reference value of the standardsample is set to N_(H2)/S_(H2). The integral value of a spectrum whenthe insulating film is subjected to the TDS analysis is denoted byS_(O2). Further, α is a coefficient which influences spectrum intensityin the TDS analysis. Refer to Japanese Published Patent Application No.H06-275697 for details of Equation 4. Note that the amount of releasedoxygen from the above insulating film can be measured, for example, witha thermal desorption spectrometer produced by ESCO Ltd., EMD-WA1000S/W,using a silicon wafer containing hydrogen atoms at 1×10¹⁶ atoms/cm² asthe standard sample.

In the TDS analysis, oxygen is partly detected as an oxygen atom. Theratio between oxygen molecules and oxygen atoms can be calculated fromthe ionization rate of the oxygen molecules. Note that, since the abovea includes the ionization rate of the oxygen molecules, the number ofthe released oxygen atoms can also be estimated through the evaluationof the number of the released oxygen molecules.

Note that N_(O2) is the number of the released oxygen molecules. Theamount of oxygen released when converted into oxygen atoms is twice thenumber of the released oxygen molecules.

For the introduction of oxygen into the oxide semiconductor layer, heattreatment performed under an oxygen atmosphere, an ion implantationmethod, an ion doping method, a plasma immersion ion implantationmethod, plasma treatment performed under an atmosphere containingoxygen, or the like can be employed.

Note that in the case where oxygen is supplied from the oxygen supplyfilm to the oxide semiconductor film by heat treatment, it is preferablethat a film having a low oxygen or water vapor permeability (alsoreferred to as a low moisture permeability) be formed under the oxygensupply film (that is, on a surface of the oxygen supply film which isopposite to its surface in contact with the oxide semiconductor film (ora surface close to the oxide semiconductor film)) so that oxygenreleased from the oxygen supply film can be supplied to the oxidesemiconductor film efficiently. For example, the insulating film 700 mayhave a stacked layer structure in which a barrier film such as analuminum oxide film, an aluminum oxynitride film, or an aluminum nitrideoxide film is formed under the above-described oxygen supply film. Inthe case of using an aluminum oxide film, the aluminum oxide filmpreferably has a high density (film density of greater than or equal to3.2 g/cm³, preferably greater than or equal to 3.6 g/cm³).

In this embodiment, a silicon oxide film is formed to a thickness of 300nm by a sputtering method and used as the insulating film 700.

Although the insulating film 700 is formed over the interlayerinsulating layer 468 in this embodiment, the insulating film 700 is notnecessarily formed as long as the above-described problem is solvedwithout the insulating film 700.

The oxide semiconductor film 702 can be formed as follows: after anoxide semiconductor film is formed by a sputtering method, a molecularbeam epitaxy (MBE) method, a CVD method, a pulse laser depositionmethod, an atomic layer deposition (ALD) method, or the like, a resistmask is formed over the oxide semiconductor film by a photolithographymethod, and part of the oxide semiconductor film is selectively etchedusing the resist mask. Note that the resist mask is removed after theoxide semiconductor film 702 is formed. The thickness of the oxidesemiconductor film is greater than 5 nm and less than or equal to 200nm, preferably greater than or equal to 10 nm and less than or equal to30 nm.

The oxygen vacancy in the oxide semiconductor film 702 is preferablyreduced as much as possible as described above. To reduce the oxygenvacancy in the oxide semiconductor film 702 as much as possible, it ispreferable that the oxide semiconductor film be formed in a depositionatmosphere in which an oxygen gas accounts for a large proportion.Therefore it can be said that an apparatus into which oxygen can beintroduced and in which the gas flow rate can be adjusted, such as asputtering apparatus, is preferably used. Further, 90% or more of thegas introduced into a deposition chamber of the sputtering apparatus isan oxygen gas, and in the case where another gas is used in addition tothe oxygen gas, a rare gas is preferably used.

Further, it is more preferable that the gas introduced into thedeposition chamber be only an oxygen gas and the percentage of an oxygengas in the deposition atmosphere be as close to 100% as possible.

As described above, the oxide semiconductor film 702 preferably containshydrogen as little as possible. Therefore the hydrogen concentration inthe oxide semiconductor film 702 is preferably less than 5×10¹⁸atoms/cm³, more preferably less than or equal to 1×10¹⁸ atoms/cm³, stillmore preferably less than or equal to 5×10¹⁷ atoms/cm³, further morepreferably less than or equal to 1×10¹⁶ atoms/cm³. Note that the abovehydrogen concentration in the oxide semiconductor film is measured bysecondary ion mass spectrometry (SIMS).

For the above-described reason, it is preferable that the gas used forthe formation of the oxide semiconductor film 702 do not contain animpurity such as water, hydrogen, a compound having a hydroxyl group, ora hydride. Alternatively, it is preferable to use a gas having a puritygreater than or equal to 6N, preferably greater than or equal to 7N(i.e., the impurity concentration in the gas is less than or equal to 1ppm, preferably less than or equal to 0.1 ppm).

In the formation of the oxide semiconductor film 702, in order to removemoisture (including water, hydrogen, a compound having a hydroxyl group,or a hydroxide) in the deposition chamber, an entrapment vacuum pumpsuch as a cryopump, an ion pump, or a titanium sublimation pump ispreferably used. The evacuation unit may be a turbo molecular pumpprovided with a cold trap. From the deposition chamber which isevacuated with a cryopump, a hydrogen atom, a compound containing ahydrogen atom such as water (H₂O) (more preferably, also a compoundcontaining a carbon atom), and the like are removed, whereby theconcentration of an impurity such as hydrogen or moisture in the oxidesemiconductor film formed in the deposition chamber can be reduced.

The oxide semiconductor film is formed in the state where the substrate(i.e., the first temporary fixing substrate 314) is held in a depositionchamber kept under reduced pressure. At this time, film formation may beperformed while the substrate is heated at a temperature greater than orequal to 100° C. and lower than or equal to the strain point of thesubstrate. By heating the substrate during the film formation, theconcentration of impurities such as hydrogen and moisture in the formedoxide semiconductor film can be reduced (this can also be referred to asdehydration treatment or dehydrogenation treatment). In addition,embrittlement due to the sputtering can be reduced, which is preferable.

Further, by subjecting the substrate to heat treatment in the depositionchamber before the oxide semiconductor film is formed on the substrate,the impurities adsorbed in the substrate can be removed. In the heattreatment, for example, the temperature of the substrate (i.e., thefirst temporary fixing substrate 314) is set greater than or equal toroom temperature and less than or equal to 450° C., preferably greaterthan or equal to 100° C. and less than or equal to 450° C. Note that byevacuation of the deposition chamber to a high vacuum, impurities can beefficiently removed in a short time.

On the other hand, when the oxide semiconductor film 702 contains analkali metal or an alkaline earth metal, the alkali metal or thealkaline earth metal and an oxide semiconductor are bonded to eachother, so that carriers are generated in some cases, which causes anincrease in the off-state current of a transistor. Accordingly, theconcentration of an alkali metal or an alkaline earth metal in the oxidesemiconductor film 702 is less than or equal to 1×10¹⁸ atoms/cm³,preferably less than or equal to 2×10¹⁶ atoms/cm³.

A target used in the sputtering apparatus preferably has a relativedensity of greater than or equal to 90%, preferably greater than orequal to 95%, further preferably greater than or equal to 99%. With theuse of a target having a high relative density, the formed oxidesemiconductor film can be a dense film.

An oxide semiconductor material used for the oxide semiconductor filmcontains indium (In). In particular, In and zinc (Zn) are preferablycontained. In addition, as a stabilizer for reducing variations inelectric characteristics of an OS transistor, gallium (Ga) is preferablycontained in addition to In and Zn. Tin (Sn) is preferably contained asa stabilizer. Hafnium (Hf) is preferably contained as a stabilizer.Aluminum (Al) is preferably contained as a stabilizer. Zirconium (Zr) ispreferably contained as a stabilizer.

As another stabilizer, one or more kinds of lanthanoid such as lanthanum(La), cerium (Ce), praseodymium (Pr), neodymium (Nd), samarium (Sm),europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium(Ho), erbium (Er), thulium (Tm), ytterbium (Yb), or lutetium (Lu) may becontained.

As the oxide semiconductor, for example, any of the following can beused: indium oxide; tin oxide; zinc oxide, an In—Zn-based oxide, anIn—Mg-based oxide, or an In—Ga-based oxide, an In—Ga—Zn-based oxide(also referred to as IGZO), an In—Al—Zn-based oxide, an In—Sn—Zn-basedoxide, an In—Hf—Zn-based oxide, an In—La—Zn-based oxide, anIn—Ce—Zn-based oxide, an In—Pr—Zn-based oxide, an In—Nd—Zn-based oxide,an In—Sm—Zn-based oxide, an In—Eu—Zn-based oxide, an In—Gd—Zn-basedoxide, an In—Tb—Zn-based oxide, an In—Dy—Zn-based oxide, anIn—Ho—Zn-based oxide, an In—Er—Zn-based oxide, an In—Tm—Zn-based oxide,an In—Yb—Zn-based oxide, or an In—Lu—Zn-based oxide, anIn—Sn—Ga—Zn-based oxide, an In—Hf—Ga—Zn-based oxide, anIn—Al—Ga—Zn-based oxide, an In—Sn—Al—Zn-based oxide, anIn—Sn—Hf—Zn-based oxide and an In—Hf—Al—Zn-based oxide.

Note that here, for example, an “In—Ga—Zn-based oxide” means an oxidecontaining In, Ga, and Zn as its main components and there is nolimitation on the ratio of In to Ga and Zn. Further, a metal elementother than the In, Ga, and Zn may be contained.

Alternatively, a material represented by 1 nMO₃(ZnO)_(m) (m>0, where mis not an integer) may be used as the oxide semiconductor. Note that Mrepresents one or more metal elements selected from Ga, Fe, Mn, and Co.Further alternatively, as the oxide semiconductor, a materialrepresented by In₂SnO₅(ZnO)_(n) (n>0, n is an integer) may be used.

The formed oxide semiconductor film 702 is in a single crystal state, apolycrystalline (also referred to as polycrystal) state, an amorphousstate, or the like.

The oxide semiconductor film 702 may be in a non-single-crystal state,for example. The non-single-crystal state is, for example, structured byat least one of c-axis aligned crystal (CAAC), polycrystal,microcrystal, and an amorphous part. The density of defect states of anamorphous part is higher than those of microcrystal and CAAC. Thedensity of defect states of microcrystal is higher than that of CAAC.Note that an oxide semiconductor including CAAC is referred to as aCAAC-OS (c-axis aligned crystalline oxide semiconductor).

For example, the oxide semiconductor film 702 may include a CAAC-OS. Inthe CAAC-OS, for example, c-axes are aligned, and a-axes and/or b-axesare not macroscopically aligned.

For example, the oxide semiconductor film 702 may include microcrystal.Note that an oxide semiconductor including microcrystal is referred toas a microcrystalline oxide semiconductor. A microcrystalline oxidesemiconductor film includes microcrystal (also referred to asnanocrystal) with a size greater than or equal to 1 nm and less than 10nm, for example.

For example, the oxide semiconductor film 702 may include an amorphouspart. Note that an oxide semiconductor including an amorphous part isreferred to as an amorphous oxide semiconductor. An amorphous oxidesemiconductor film, for example, has disordered atomic arrangement andno crystalline component. Alternatively, an amorphous oxidesemiconductor film is, for example, absolutely amorphous and has nocrystal part.

Note that the oxide semiconductor film 702 may be a mixed film includingany of a CAAC-OS, a microcrystalline oxide semiconductor, and anamorphous oxide semiconductor. The mixed film, for example, includes aregion of an amorphous oxide semiconductor, a region of amicrocrystalline oxide semiconductor, and a region of a CAAC-OS.Further, the mixed film may have a stacked structure including a regionof an amorphous oxide semiconductor, a region of a microcrystallineoxide semiconductor, and a region of a CAAC-OS, for example.

Note that, as mentioned above, the oxide semiconductor film 702 may havea single-crystal, for example.

The oxide semiconductor film 702 preferably includes a plurality ofcrystal parts. In each of the crystal parts, a c-axis is preferablyaligned in a direction parallel to a normal vector of a surface wherethe oxide semiconductor film is formed or a normal vector of a surfaceof the oxide semiconductor film. Note that, among crystal parts, thedirections of the a-axis and the b-axis of one crystal part may bedifferent from those of another crystal part. An example of such anoxide semiconductor film is a CAAC-OS film.

The CAAC-OS film is not absolutely amorphous. Note that in most cases,the crystal part fits inside a cube whose one side is less than 100 nm.In an image obtained with a transmission electron microscope (TEM), aboundary between an amorphous part and a crystal part and a boundarybetween crystal parts in the CAAC-OS film are not clearly detected.Further, with the TEM, a grain boundary in the CAAC-OS film is notclearly found. Thus, in the CAAC-OS film, a reduction in electronmobility due to the grain boundary is suppressed.

In each of the crystal parts included in the CAAC-OS film, a c-axis isaligned in a direction parallel to a normal vector of a surface wherethe CAAC-OS film is formed or a normal vector of a surface of theCAAC-OS film. Further, in each of the crystal parts, metal atoms arearranged in a triangular or hexagonal configuration when seen from thedirection perpendicular to the a-b plane, and metal atoms are arrangedin a layered manner or metal atoms and oxygen atoms are arranged in alayered manner when seen from the direction perpendicular to the c-axis.Note that, among crystal parts, the directions of the a-axis and theb-axis of one crystal part may be different from those of anothercrystal part. In this specification, a term “perpendicular” includes arange from 80° to 100° or from 85° to 95°. In addition, a term“parallel” includes a range from −10° to 10° or from −5° to 5°.

In the CAAC-OS film, distribution of crystal parts is not necessarilyuniform. For example, in the formation process of the CAAC-OS film, inthe case where crystal growth occurs from a surface side of the oxidesemiconductor film, the proportion of crystal parts in the vicinity ofthe surface of the oxide semiconductor film is higher than that in thevicinity of the surface where the oxide semiconductor film is formed insome cases. Further, when an impurity is added to the CAAC-OS film, thecrystal part in a region to which the impurity is added becomesamorphous in some cases.

Since the c-axes of the crystal parts included in the CAAC-OS film arealigned in the direction parallel to a normal vector of a surface wherethe CAAC-OS film is formed or a normal vector of a surface of theCAAC-OS film, the directions of the c-axes may be different from eachother depending on the shape of the CAAC-OS film (the cross-sectionalshape of the surface where the CAAC-OS film is formed or thecross-sectional shape of the surface of the CAAC-OS film). Note that thefilm formation is accompanied with the formation of the crystal parts orfollowed by the formation of the crystal parts through crystallizationtreatment such as heat treatment. Hence, the c-axes of the crystal partsare aligned in the direction parallel to a normal vector of the surfacewhere the CAAC-OS film is formed or a normal vector of the surface ofthe CAAC-OS film.

In a transistor using the CAAC-OS film, change in electriccharacteristics due to irradiation with visible light or ultravioletlight is small. Thus, the transistor has high reliability.

Note that part of oxygen included in the oxide semiconductor film may besubstituted with nitrogen.

In an oxide semiconductor having a crystal part such as the CAAC-OSfilm, defects in the bulk can be further reduced and when the surfaceflatness of the oxide semiconductor is improved, mobility higher thanthat of an oxide semiconductor in an amorphous state can be obtained. Toimprove the surface flatness, the oxide semiconductor is preferablyformed over a flat surface. Specifically, the oxide semiconductor ispreferably formed over a surface with an average surface roughness(R_(α)) less than or equal to 1 nm, more preferably less than or equalto 0.3 nm, still more preferably less than or equal to 0.1 nm. Thus,planarization treatment is preferably performed on a surface over whichthe oxide semiconductor is to be formed. As the planarization treatment,chemical mechanical polishing (CMP) treatment, a dry etching method, orthe like may be used. As the planarization treatment, for example,reverse sputtering, a dry etching method, chemical mechanical polishing(CMP) treatment, or the like can be used. The “reverse sputtering” hererefers to a method in which, without application of a voltage to atarget side, high frequency power is applied to a substrate side in anatmosphere of argon, nitrogen, helium, oxygen, or the like to generateplasma in the vicinity of the substrate and modify a surface (here, theoxide semiconductor film).

In the case where CMP treatment is employed as the above planarizationtreatment, it may be performed only once or more than once. When the CMPtreatment is performed more than once, first polishing is preferablyperformed with a high polishing rate followed by final polishing with alow polishing rate. By performing polishing steps with differentpolishing rates in combination, the planarity of the surface over whichthe oxide semiconductor is to be formed can be further improved.

Note that “average surface roughness (R_(α))” described above isobtained by expanding arithmetic mean surface roughness, which isdefined by JIS B0601:2001 (ISO4287:1997), into three dimensions so as tobe applied to a surface. In addition, R_(α) can be expressed as an“average value of the absolute values of deviations from a referencesurface to a specific surface” and is defined by the following equation.

$\begin{matrix}{R_{a} = {\frac{1}{S_{0}}{\int_{y\; 1}^{y\; 2}{\int_{x\; 1}^{x\; 2}{{{{f\left( {x,y} \right)} - Z_{0}}}\ {x}{y}}}}}} & (5)\end{matrix}$

Here, the specific surface is a surface which is a target of roughnessmeasurements, and is a quadrilateral region which is specified by fourpoints represented by the coordinates (x₁, y₁,f(x₁, y₁)), (x₁, y₂,f(x₁,y₂)), (x₂, y₁, f(x₂, y₁)), and (x₂, y₂, f(x₂, y₂)). Further, S₀represents the area of a rectangle which is obtained by projecting thespecific surface on the x-y plane, and Z₀ represents the height of thereference surface (the average height of the specific surface). Notethat R_(a) can be measured using an atomic force microscope (AFM).

In the case where the CAAC-OS film is formed as the oxide semiconductorfilm, any of the following three methods may be employed. The firstmethod is the one in which the oxide semiconductor film is formed at atemperature higher than or equal to 200° C. and lower than or equal to450° C., so that the oxide semiconductor film 702 serves as the CAAC-OSfilm. The second method is the one in which the oxide semiconductor film702 is formed and then subjected to heat treatment at a temperaturehigher than or equal to 200° C. and lower than or equal to 700° C., sothat the oxide semiconductor film 702 serves as the CAAC-OS film. Thethird method is the one in which two layers are formed for formation ofan oxide semiconductor film. That is, after a first oxide semiconductorfilm with a small thickness is formed, heat treatment is performed at atemperature higher than or equal to 200° C. and lower than or equal to700° C., so that the first oxide semiconductor film serves as a CAAC-OSfilm. Then, a second oxide semiconductor film is formed over the firstoxide semiconductor film using a crystal in the first oxidesemiconductor film as a seed crystal, whereby the CAAC-OS film isobtained.

Over the oxide semiconductor film (preferably a CAAC-OS film) formed byany of the above methods, a resist mask is formed by a photolithographymethod, and selective etching is performed using the resist mask. Thus,the island-shaped oxide semiconductor film 702 is formed.

Note that after the oxide semiconductor film is formed or after theisland-shaped oxide semiconductor film 702 is formed, “dehydrationtreatment (dehydrogenation treatment)” and “peroxidation treatment”described below may be performed on the oxide semiconductor film (or theoxide semiconductor film 702).

<Dehydration (Dehydrogenation) Treatment>

As dehydration treatment (dehydrogenation treatment), a substrate issimply heated at a temperature greater than or equal to 300° C. andlower than or equal to 700° C., or less than the strain point of thesubstrate. By the heat treatment, excess hydrogen (including water and ahydroxyl group) can be removed.

As a heat treatment apparatus, it is possible to use an apparatus forheating an object to be processed using thermal conduction or thermalradiation generated from a medium such as a heated gas. For example, anelectric furnace and a rapid thermal anneal (RTA) apparatus such as agas rapid thermal anneal (GRTA) apparatus or a lamp rapid thermal anneal(LRTA) apparatus can be used. An LRTA apparatus is an apparatus forheating an object to be processed by radiation of light (electromagneticwave) emitted from a lamp such as a halogen lamp, a metal halide lamp, axenon arc lamp, a carbon arc lamp, a high pressure sodium lamp, or ahigh pressure mercury lamp. A GRTA apparatus is an apparatus forperforming heat treatment using a high-temperature gas. As the gas, aninert gas which does not react with an object to be processed by heattreatment, such as nitrogen or a rare gas such as argon is used.

For example, as the heat treatment, GRTA process may be performed asfollows. The object to be processed is put in a heated inert gasatmosphere, heated for several minutes, and taken out of the inert gasatmosphere. The GRTA process enables high-temperature heat treatment fora short time. Moreover, the GRTA process can be employed even when thetemperature exceeds the upper temperature limit of the object to beprocessed. Note that the inert gas may be switched to a gas containingoxygen during the process.

Note that as the inert gas atmosphere, an atmosphere that containsnitrogen or a rare gas (e.g., helium, neon, or argon) and does notcontain water, hydrogen, or the like is preferably used. For example,the purity of nitrogen or a rare gas such as helium, neon, or argonintroduced into a heat treatment apparatus is greater than or equal to6N (99.9999%), preferably greater than or equal to 7N (99.99999%) (thatis, the concentration of the impurities is less than or equal to 1 ppm,preferably less than or equal to 0.1 ppm).

<Peroxidation Treatment>

The dehydration or dehydrogenation treatment may be accompanied byelimination of oxygen which is a component for an oxide semiconductorfilm to lead to a reduction in oxygen. Thus, in the case where thedehydration or dehydrogenation treatment is performed, oxygen ispreferably supplied to the oxide semiconductor film. That is, it ispreferred to perform the peroxidation treatment.

The oxygen vacancy in the oxide semiconductor film may be compensated inthe following manner, for example: after the oxide semiconductor film issubjected to the dehydration treatment (dehydrogenation treatment), ahigh-purity oxygen gas, a high-purity dinitrogen monoxide gas, ahigh-purity nitrous oxide gas, or ultra dry air (the moisture amount isless than or equal to 20 ppm (−55° C. by conversion into a dew point),preferably less than or equal to 1 ppm, more preferably less than orequal to 10 ppb estimated by the use of a dew-point instrument of acavity ring down laser spectroscopy (CRDS) system) may be introducedinto the same furnace. It is preferable that the oxygen gas or thedinitrogen monoxide gas do not contain water, hydrogen, and the like.Alternatively, the purity of the oxygen gas or the dinitrogen monoxidegas which is introduced into the heat treatment apparatus is preferably6N or higher, further preferably 7N or higher (i.e., the impurityconcentration in the oxygen gas or the dinitrogen monoxide gas ispreferably 1 ppm or lower, further preferably 0.1 ppm or lower). Notethat the pressure in the apparatus is set high in the heat treatment,whereby oxygen can be efficiently added to the oxide semiconductor film.

Alternatively, the oxygen vacancy in the oxide semiconductor film (orthe oxide semiconductor film 702) can be compensated in the followingmanner, for example: oxygen (including at least one of an oxygenradical, an oxygen atom, and an oxygen ion) is added to the oxidesemiconductor film by an ion implantation method, an ion doping method,a plasma immersion ion implantation method, plasma treatment, or thelike.

As described above, the formed oxide semiconductor film is subjected tothe dehydration treatment (dehydrogenation treatment), whereby hydrogenor moisture is removed from the oxide semiconductor film so that theoxide semiconductor film is purified so as to contain impurities aslittle as possible. Then, oxygen which is simultaneously reduced issupplied, whereby the oxygen vacancy can be repaired.

In this embodiment, a 15-nm-thick IGZO film is formed using theabove-mentioned gas including less hydrogen by a sputtering method whilethe substrate temperature is held at a temperature of 200° C., so that aCAAC-OS film is formed. After dehydrogenation of the film under anitrogen gas atmosphere at 350° C. for one hour, the film is subjectedto treatment for making an oxygen-excess state under a mixed gasatmosphere of nitrogen and oxygen at 350° C. for one hour and processedto have an island shape. This film is used as the oxide semiconductorfilm 702.

Next, over the oxide semiconductor film 702, a conductive film 704electrically connected to the oxide semiconductor film 702, a gateinsulating film 706 covering part of the oxide semiconductor film 702,and a gate electrode 708 overlapping with the oxide semiconductor film702 with the gate insulating film 706 interposed therebetween areformed. Then, an impurity ion having the function of reducing theresistance of the oxide semiconductor film 702 is added to the oxidesemiconductor film 702 with use of the gate electrode 708 as a mask, sothat a low-resistance region 702 a and a channel formation region 702 bare formed in the oxide semiconductor film 702. Thus, a transistor 720and a transistor 730 including the oxide semiconductor film as an activelayer are formed (see FIG. 6A).

The conductive film 704, the gate insulating film 706, and the gateelectrode 708 can be formed using a method and a material similar tothose of the gate electrode 404 a, the insulating film 401, and the gateelectrode 404 a, respectively.

For the conductive film 704 in contact with the oxide semiconductor film702, it is preferable to use a single film structure or a stacked filmstructure containing a tungsten film, a titanium film, or a molybdenumfilm, for example; in particular, a tungsten film is preferably used.

For example, a stacked film containing a tungsten film and a titaniumfilm over the tungsten film can be use as the conductive film 704.

One or more selected from the following can be used as the impurity ionadded to the above oxide semiconductor film 702: Group 15 elements(typified by nitrogen (N), phosphorus (P), arsenic (As), and antimony(Sb)), boron (B), aluminum (Al), argon (Ar), helium (He), neon (Ne),indium (In), fluorine (F), chlorine (Cl), titanium (Ti), and zinc (Zn).Note that the addition of the impurity ion is not necessarily performed.

In this embodiment, a 100-nm-thick tungsten film formed by a sputteringmethod is used as the conductive film 704. A 20-nm-thick silicon oxidefilm formed with plasma generated by a microwave is used as the gateinsulating film 706. A stacked layer film including a 30-nm-thicktantalum nitride film and a 135-nm-thick tungsten film which are formedby a sputtering method is used as the gate electrode 708.

Then, an interlayer insulating layer 740 including an interlayerinsulating film 740 a and an interlayer insulating film 740 b is formedover the transistor 720 and the transistor 730, so that the second layer112 is formed (see FIG. 6B).

The interlayer insulating layer 740 can be formed using a method and amaterial similar to those of the interlayer insulating layer 450, theinsulating film 401, or the like.

Further, before the interlayer insulating layer 740 is formed, forexample, a conductive film 711 can be formed in part of a region overthe conductive film 704 with the gate insulating film 706 interposedtherebetween to form a capacitor 712, so that the second layer 112including the latch circuits illustrated in FIG. 1B can be formed (seeFIG. 6B).

In this embodiment, a 70-nm-thick aluminum oxide film formed by asputtering method is used as the interlayer insulating film 740 a and a300-nm-thick silicon oxide film formed by a sputtering method is used asthe interlayer insulating film 740 b.

By the above-described process, the circuit layer 110 having the firstlayer 111 including the transistor including a single crystalsemiconductor film as an active layer and the second layer 112 includingthe OS transistor is formed over the first temporary fixing substrate314 with the separation layer 310 interposed between the first temporaryfixing substrate 314 and the circuit layer 110 (see FIG. 6B).

Over the second layer 112, it is preferable to form a conductive layer716 a, which is electrically connected to a component (e.g., transistor)included in the second layer 112, and a conductive layer 716 b, which isnot in direct contact with the conductive layer 716 a (see FIG. 7). Withthis structure, the signal processing unit 100 including the circuitlayer 110 and formed in a later process can be easily mounted on anotherdevice (e.g., a display device) by electrically connecting theconductive layers 716 a and 716 b to a conductive layer (e.g., a leadwiring) exposed over a surface of the device using a conductivematerial. Note that the conductive layers 716 a and 716 b may beconstrued as being included in the circuit layer 110 as illustrated inFIG. 7.

Any of the materials that can be used for the gate electrode 404 a canbe used for the conductive layers 716 a and 716 b. In this embodiment,as the conductive layers 716 a and 716 b, a stacked layer film includinga 50-nm-thick titanium film, a 200-nm-thick aluminum film, and a50-nm-thick titanium film which are formed in this order by a sputteringmethod is used. The conductive layers 716 a and 716 b may be formed to athickness from 1 μm to 20 μm. In this case, the conductive layers 716 aand 716 b may be formed with Cu or an alloy containing Cu by a goldimpregnation method or may be formed in such a way that a pastecontaining Ag is printed by a screen printing method and then hardened.

<Method of Manufacturing Signal Processing Unit 100>

Next, a method in which the above-described circuit layer 110 is formedover a flexible substrate to manufacture the signal processing unit 100is described using FIGS. 8A to 8C. Note that in FIGS. 8A to 8C, thedetailed structure of the elements included in the first layer 111 andthe second layer 112 is not shown to avoid complication.

First, with use of a temporary fixing material 600, a second temporaryfixing substrate 602 is bonded onto the circuit layer 110 formed asdescribed above over the first temporary fixing substrate 314 with theseparation layer 310 interposed therebetween (see FIG. 8A).

As the temporary fixing material 600, a material which can separate thesecond temporary fixing substrate 602 from the circuit layer 110 asnecessary, such as a material which is soluble to water or an organicsolvent or which can be plasticized by ultraviolet light irradiation, isused.

The temporary fixing material 600 is preferably formed to be thin andhave a uniform thickness using any of coating machines such as a spincoater, a slit coater, a gravure coater, and a roll coater, or any ofprinting machines such as a flexographic printing machine, an offsetprinting machine, a gravure printing machine, a screen printing machine,and an inkjet machine.

As the second temporary fixing substrate 602, a tape whose adhesion of asurface can be arbitrarily decreased, such as a UV separation tape and athermal separation tape, can be used. Alternatively, a glass substrate,a quartz substrate, a sapphire substrate, a ceramic substrate, a metalsubstrate, a plastic substrate, or the like may be used. Note that inthe case where the tape whose adhesion of a surface can be arbitrarilydecreased is used, the temporary fixing material 600 is not necessarilyrequired additionally. In the case where a plastic substrate is used, aplastic substrate having heat resistance high enough to withstand thetemperature of a process performed later is preferably used.

Note that there is no particular limitation on the method of bonding thesecond temporary fixing substrate 602 to the circuit layer 110. When aflexible material such as the tape is used as the second temporaryfixing substrate 602, a device which can perform bonding using a roller(also referred to as a roll laminator) may be used, for example, so thatthe circuit layer 110 and the second temporary fixing substrate 602 canbe stably bonded to each other without the formation of air bubbles andthe like therebetween.

In this embodiment, an adhesive which is cured by ultraviolet lightirradiation and is soluble to water even after curing (also referred toas a water-soluble adhesive or the like) is used as the temporary fixingmaterial 600, and is applied to a surface of the circuit layer 110 witha spin coating apparatus to form its thin layer, and curing treatment isperformed. After that, a UV separation tape as the second temporaryfixing substrate 602 is bonded onto to the temporary fixing material 600with the use of a roll laminator.

Next, the circuit layer 110 is separated from the first temporary fixingsubstrate 314 and transferred to the second temporary fixing substrate602 (see FIG. 8B).

When any of the layers included in the separation layer 310 is a metaloxide film (e.g., in this embodiment, the second separation layer 308 isa tungsten oxide film which is a metal oxide film), the metal oxide filmis crystallized and embrittled by heat treatment performed at severaltimes in the formation of the circuit layer 110. Hence, by applicationof a physical force (e.g., a force to separate the first temporaryfixing substrate 314 from the second temporary fixing substrate 602),the circuit layer 110 can be easily transferred from the first temporaryfixing substrate 314 to the second temporary fixing substrate 602 withthe separation layer 310 used as an interface.

In FIG. 8B, part of the separation layer (part of the second separationlayer 308 or the oxide film 309) remains in contact with the circuitlayer 110. Part of or the whole of the separation layer remaining incontact with the circuit layer 110 may be removed by dry etchingtreatment, wet etching treatment, or chemical mechanical polishingtreatment. Note that the part of the separation layer remaining on thecircuit layer 110 may be construed as being included in the circuitlayer 110.

Next, after the base substrate 115 is bonded to the circuit layer 110using a bond material 604, the temporary fixing material 600 and thesecond temporary fixing substrate 602 are separated from the basesubstrate 115 (see FIG. 8C).

As the bond material 604, any of a variety of curable adhesives, forexample, a photo-curable adhesive such as a UV curable adhesive, areactive curable adhesive, a thermosetting adhesive, and an anaerobicadhesive can be used. The bond material 604 can be formed to be thin andhave a uniform thickness using any of coating machines such as a spincoater, a slit coater, a gravure coater, and a roll coater, or any ofprinting machines such as a flexographic printing machine, an offsetprinting machine, a gravure printing machine, a screen printing machine,and an inkjet machine.

As the base substrate 115, a flexible substrate can be used. Forexample, an organic resin substrate (which can be referred to as anorganic resin thin film in consideration of its flexibility), or a metalsubstrate (which can be referred to as a metal thin film inconsideration of its flexibility) is used. Accordingly, the drivercircuit can be flexibly deformed along a curved surface or an appliedstress and thus hardly causes a breakdown or a peeling due to a shock orapplication of a stress in bending or twisting. Note that, whenflexibility is not required, the substrate explained for the firsttemporary fixing substrate 314 can be used as the base substrate 115.

As the organic resin substrate, for example, a substrate including, ascomponents, one or more kinds of resins selected from a poly(ethyleneterephthalate) (PET) resin, a poly(ether sulfone) (PES) resin, apoly(ethylene naphthalate) (PEN) resin, a poly(vinyl alcohol) (PVA)resin, a polycarbonate (PC) resin, a nylon resin, an acrylic resin, apolyacrylonitrile resin; a polyetheretherketone (PEEK) resin, apolystyrene (PS) resin, a polysulfone (PSF) resin, a polyetherimide(PEI) resin, a polyarylate (PAR) resin, a poly(butylene terephthalate)(PBT) rein, a polyimide (PI) resin, a polyamide (PA) resin, a poly(amideimide) (PAI) resin, a polyisobutylene (PIE) resin, a chlorinatedpolyether (CP) resin, a melamine (MF) resin, an epoxy (EP) resin, apoly(vinylidene chloride) (PVdC) resin, a polypropylene (PP) resin, apolyacetal (POM) resin, a fluorine resin, a phenol (PF) resin, a furanresin (FF), an unsaturated polyester (FRP) resin, a cellulose acetate(CA) resin, a urea (UF) resin, a xylene (XR) resin, a poly(diallylphthalate) (DAP) resin, a poly(vinyl acetate) (PVAc) resin, apolyethylene (PE) resin, and an ABS resin can be used. Further, amaterial in which a glass fiber, an aramid fiber, a carbon fiber, or thelike impregnated with any of the above resins (also referred to as aprepreg or the like) can also be used.

As the above metal substrate, for example, aluminum (Al), titanium (Ti),nickel (Ni), chromium (Cr), molybdenum (Mo), tantalum (Ta), berylium(Be), zirconium (Zr), gold (Au), silver (Ag), copper (Cu), zinc (Zn),iron (Fe), lead (Pb), or tin (Sn), or a substrate including an alloycontaining any of these elements can be used.

Note that, before bonding to the circuit layer 110, the base substrate115 is preferably subjected to fluid jet cleaning, ultrasonic cleaning,plasma cleaning, UV cleaning, ozone cleaning, or the like so that dustand organic components attaching to the base substrate 115 are removed.

Further, before bonding to the circuit layer 110, heat treatment may beperformed on the base substrate 115. By the heat treatment, moisture andimpurities attaching to the base substrate 115 can be removed. Further,by the heat treatment under a reduced pressure, moisture and impuritiescan be removed more efficiently.

Note that as for the cleaning method and the heat treatment, any one ofthe above cleaning methods and the heat treatment may be selected or twoor more of the heat treatment and the cleaning methods may be performedin combination. For example, after fluid jet cleaning is performed toremove dust attaching to the base substrate 115, ozone cleaning isperformed to remove organic components, and then heat treatment isperformed lastly to remove moisture attaching to and moisture absorbedin the base substrate 115 when the fluid jet cleaning is performed. Insuch a manner, dust, organic components and moisture the base substrate115 can be effectively removed.

In this embodiment, a film formed by impregnation of an aramid fiberwith a polyimide resin is used as the base substrate 115 and a thinthermosetting adhesive is provided on a surface of the film using ascreen printing machine to form the bond material 604. Then, the basesubstrate 115 and the circuit layer 110 are bonded to each other andsubjected to hardening treatment.

Through the above-described process, the signal processing unit 100 inwhich the circuit layer 110 including the first layer 111 including thetransistor including a single crystal semiconductor film as an activelayer and the second layer 112 including the OS transistor r is providedover the base substrate 115 and which is illustrated in FIG. 1A, isformed (see FIG. 8C).

In the signal processing unit 100, the OS transistors are used for thelatch circuits in the circuit layer 110. Since the off-state current ofthe OS transistors is extremely low, power consumption in the latchcircuits can be reduced particularly in the case where the same signalis successively input to the latch circuits. A component other than thelatch circuits which are included in the circuit layer 110 includes thetransistor including a single crystal semiconductor film as an activelayer, which ensures the high-speed driving performance of the circuitlayer 110. Since the circuit layer 110 is formed over the flexiblesubstrate, the signal processing unit 100 is a driver circuit thatachieves high-speed driving performance and low power consumption andhardly causes a breakdown or a peeling due to a shock or application ofa stress in bending and twisting.

Embodiment 2

In this embodiment, a method to manufacture the signal processing unit100, which is different from the method in Embodiment 1, is described.

First, as in Embodiment 1, after the insulating film 302 is formed overa surface of the semiconductor substrate 300, the ion irradiationtreatment 304 is performed from one side of the semiconductor substrate300, so that the embrittled region 306 is formed in the semiconductorsubstrate 300 (see FIG. 24A).

Next, the separation layer 310 is formed over the surface of the firsttemporary fixing substrate 314. Note that the separation layer 310 is astacked layer structure including the first separation layer 307, thesecond separation layer 308, and the oxide film 309 in FIG. 24B but isnot limited to this structure as long as the structure is mentioned forthe separation layer 310 in Embodiment 1.

Next, the first temporary fixing substrate 314 whose surface is providedwith the separation layer 310 is bonded to the side of the semiconductorsubstrate 300 (side on which the ion irradiation treatment is performed)so that the side of the semiconductor substrate 300 is in contact withthe separation layer 310 of the first temporary fixing substrate 314(see FIG. 24C).

Note that the adhesion between the semiconductor substrate 300 and thefirst temporary fixing substrate 314 tends to be stronger as theflatness of the surfaces of the insulating film 302 and the separationlayer 310 becomes higher. Therefore, before the semiconductor substrate300 and the first temporary fixing substrate 314 are bonded to eachother, one or both of the insulating film 302 and the separation layer310 is preferably subjected to planarization treatment. For theplanarization treatment, refer to the planarization treatment describedin Embodiment 1.

Next, as in Embodiment 1, after the semiconductor substrate 300 issubjected to heat treatment, the semiconductor substrate 300 isseparated from the first temporary fixing substrate 314.

By the above-described process, the semiconductor thin film 316separated from the semiconductor substrate 300 can be transferred ontothe first temporary fixing substrate 314 with the insulating film 302and the separation layer 310 interposed therebetween (see FIG. 24D).

The stacking order of the layers in the separation layer 310 formed overthe first temporary fixing substrate 314 described in this embodiment isopposite to that in the separation layer 310 described in Embodiment 1(in Embodiment 1, the first separation layer 307, the second separationlayer 308, and the oxide film 309 are formed in this order over thefirst temporary fixing substrate 314, while, in this embodiment, theoxide film 309, the second separation layer 308, and the firstseparation layer 307 are stacked in this order over the first temporaryfixing substrate 314). However, the signal processing unit 100 can bemanufactured with either structure, and the separation layer havingeither structure is referred to as the separation layer 310.

For the subsequent steps in the manufacturing process of the signalprocessing unit 100, refer to FIG. 3D, FIGS. 4A to 4D, FIGS. 5A and 5B,FIGS. 6A and 6B, and FIG. 7 and the description with reference to thesedrawings in Embodiment 1.

By using the manufacturing method described in this embodiment,formation of the insulating film 302 on the surface of the semiconductorsubstrate 300, formation of the embrittled region 306 in thesemiconductor substrate 300, and formation of the separation layer 310over the first temporary fixing substrate 314 can be performed at thesame time with different apparatuses, which can shorten the time formanufacturing the signal processing unit 100.

Embodiment 3

In this embodiment, examples of a structure and a manufacturing methodof a display device including the signal processing unit described inthe above embodiments is described with reference to FIGS. 9A to 9C andFIGS. 10A to 10D.

<Structure Example of Display Device>

FIG. 9A shows a top view of an example of the display device of thisembodiment. FIG. 9B illustrates a section along the alternate long andshort dash line A1-A2 in FIG. 9A. FIG. 9C illustrates a section alongthe alternate long and short dash line B1-B2 in FIG. 9A.

As illustrated in FIGS. 9A to 9C, a display device 750 described in thisembodiment includes, over a substrate 760, a pixel portion 762 includinga thin film transistor and a pixel electrode, a counter substrate 770bonded to the substrate with a sealing material 768, and a displayelement layer 769, which includes a liquid crystal element, an ELelement, or the like and is provided in a space enclosed by thesubstrate 760, the counter substrate 770, and the sealing material 768.Note that although the display element layer 769 is provided in thewhole space enclosed by the substrate 760, the counter substrate 770,and the sealing material 768 in FIG. 9A, a space may be present betweenthe display element layer 769 and the counter substrate 770 or amaterial that can suppress entrance of moisture from the outside (e.g.,a drying agent) may be provided in a space between the display elementlayer 769 and the counter substrate 770. In FIG. 9A, for easierunderstanding of the structure under the counter substrate 770, hatchingfor indicating the counter substrate 770 is not illustrated.

A lead terminal 764 for inputting an image signal and a power supplyvoltage is provided over the substrate 760 and electrically connected tothe circuit layer 110 in the signal processing unit 100 through aconductive material 771 a.

A flexible printed wiring 772 (flexible printed circuit: FPC) forsupplying a power supply voltage to the pixel portion 762 iselectrically connected to a connection terminal 766 provided over thesubstrate 760 through the conductive material 771 b. The connectionterminal 766 is electrically connected to the circuit layer included inthe signal processing unit 100 through the conductive material 771 a.

Thus, in the display device 750 illustrated in FIGS. 9A to 9C, thesignal processing unit 100 functions as a scan line driver circuit or asignal line driver circuit of the pixel portion 762, and an electricpower voltage is supplied from the flexible printed wiring 772 to thepixel portion 762 through the lead terminal 764, the connection terminal766, and the conductive materials 771 a and 771 b.

Although the power supply voltage supplied through the flexible printedwiring 772 is supplied to the signal processing unit 100 and then to thepixel portion 762 in this embodiment, the power supply voltage may besupplied to the pixel portion 762 and then to the signal processing unit100 or may be supplied to the signal processing unit 100 and the pixelportion 762 in parallel.

<Method of Manufacturing Display Device>

Next, an example of a manufacturing process of the display device 750illustrated in FIGS. 9A to 9C is described using FIGS. 10A to 10D.

First, the pixel portion 762, the lead terminal 764, and the connectionterminal 766 are formed over the substrate 760 (see FIG. 10A).

For the substrate 760, any of the materials that can be used for thefirst temporary fixing substrate 314 and the base substrate 115described in Embodiment 1 can be used. Since the signal processing unit100 may have flexibility as described in Embodiment 1, it is possible toprovide a structure where not only the signal processing unit 100 butalso the substrate 760 including the pixel portion 762 can be deformedin accordance with an applied stress, which enables a display devicewith flexibility as a whole and a high added value.

On the other hand, even when a material with low flexibility, such assilicon or glass is used for the substrate 760, the use of a flexiblebase substrate 115 allows the signal processing unit 100 to be thin andlightweight, which contributes to a reduction in the thickness or weightof the display device 750.

As the structure, material, formation method of the pixel portion 762, aknown technique of a pixel portion including a thin film transistor anda pixel electrode can be used without particular limitation.

A known technique can be employed to form the lead terminal 764 and theconnection terminal 766. However, in consideration of time or costrequired for the manufacture of the display device, the lead terminal764 and the connection terminal 766 are preferably formed at the sametime as the pixel portion 762 with the use of the material and formationmethod used in the manufacturing process of the pixel portion 762.

Next, while the display element layer 769 is formed over the pixelportion 762, the counter substrate 770 is bonded to the substrate 760with the sealing material 768 provided on an outer side than the displayelement layer 769 (at the edge portion of the substrate) (see FIG. 10B).

As the sealing material 768, any of a variety of curable adhesives, forexample, a photo-curable adhesive such as a UV curable adhesive, areactive curable adhesive, a thermosetting adhesive, and an anaerobicadhesive can be used. The sealing material 768 can be formed with such amaterial using any of printing machines such as a flexographic printingmachine, an offset printing machine, a gravure printing machine, ascreen printing machine, an inkjet machine, and a dispensing machine.Note that the sealing material 768 may include a spacer material.

In the display element layer 769, a liquid crystal element or anelectroluminescent (EL) element is provided by a known technique. Forexample, in the case where the display device 750 is a liquid crystaldisplay device, over the pixel electrode included in the pixel portion762, an alignment film, a liquid crystal material, an electrode and acolor filter which face the pixel electrode with the liquid crystalmaterial interposed therebetween, and the like are provided. In the casewhere the display device 750 is an EL display device, over the pixelelectrode included in the pixel portion, an EL layer including anorganic compound or an inorganic compound having a light-emittingproperty, electrodes which face the pixel electrode with the EL layerinterposed therebetween, and the like are provided. In addition, in thecase of a liquid crystal display device, a backlight or a polarizingplate may be provided as necessary.

For the counter substrate 770, any of the materials that can be used forthe first temporary fixing substrate 314 and the base substrate 115described in Embodiment 1 can be used. Therefore, a flexible substratecan be used for the counter substrate 770. However, in the case wherethe display device 750 emits light upward (in the arrow direction inFIG. 10B), for the counter substrate 770, it is necessary to use amaterial that transmits visible light (e.g., light at a wavelength from350 nm to 700 nm) (specifically, the property of transmitting 50% ormore of light, 70% or more of light).

Next, using the conductive material 771 a, the signal processing unit100 in which the circuit layer 110 is provided over the base substrate115 is provided (mounted) over the substrate 760 so that the circuitlayer 110 faces the substrate 760 (see FIG. 10C). Consequently, thecircuit layer 110 is electrically connected to the lead terminal 764 andto the connection terminal 766 through the conductive material 771 a.

As the conductive material 771 a, an anisotropic conductive paste (ACP),an anisotropic conductive film (ACF), or the like can be used and, bypressure bonding, the circuit layer 110 can be electrically connected tothe lead terminal 764 and to the connection terminal 766. Alternatively,a conductive adhesive such as a silver paste, a copper paste, or acarbon paste may be used. Alternatively, a solder material which canmelt at a temperature less than or equal to the glass transitiontemperature of the base substrate 115 or the substrate 760 may be used.

Next, the flexible printed wiring 772 is electrically connected to theconnection terminal 766 with the use of the conductive material 771 b(see FIG. 10D).

The treatment for mounting the signal processing unit 100 and thetreatment for bonding the flexible printed wiring 772 are differentprocesses in the above example. However, when the height of the signalprocessing unit 100 and the height of the flexible printed wiring 772are about the same as illustrated in FIG. 10D, the treatment formounting the signal processing unit 100 and the treatment for bondingthe flexible printed wiring 772 may be performed in the same process.

Through the above-described process, the display device 750 illustratedin FIGS. 9A and 9B is completed. In the display device 750, the pixelportion 762 provided over the substrate 760 is electrically connectedthrough the lead terminal 764 to the signal processing unit 100described in Embodiment 1. The signal processing unit 100 achieveshigh-speed driving performance and low power consumption and hardlycauses a breakdown or a peeling due to a shock or application of astress in bending or twisting. The signal processing unit 100 functionsas a scan line driver circuit or a signal line driver circuit of thepixel portion 762. Thus, a display device that achieves high-speeddriving performance and low power consumption can be provided.

When a material having flexibility is used for the substrate 760 and thebase substrate 115, not only the signal processing unit 100 but also thesubstrate 760 can be deformed in accordance with an applied stress,which enables a display device with flexibility as a whole and a highadded value.

On the other hand, even when a material with low flexibility, such asglass, is used for the substrate 760, the use of the flexible basesubstrate 115 contributes to reduction in the thickness or weight of thedisplay device 750.

Embodiment 4

In this embodiment, a structure and a feature of a display device whichis partly different from the display device 750 described in Embodiment3 are described. Note that although a manufacturing method is basicallysimilar to that in Embodiment 3, part of the method which is differentfrom that in Embodiment 3 is as appropriate described.

<Structure Example and Feature of Display Device>

A structure of a display device which is different from that inEmbodiment 3 is described below using FIGS. 11A to 11C. FIG. 11A is atop view of the display device, FIG. 11B is a cross-sectional view takenalong the alternate long and short dash line C1-C2 in FIG. 11A, and FIG.11C is a cross-sectional view taken along the alternate long and shortdash line D 1-D2 in FIG. 11A. Note that FIG. 11A is the same as FIG. 9Abut is given here for easier understanding of positional relationship ofthe components in FIG. 11B.

A display device 1050 illustrated in FIGS. 11A and 11B has the samecomponents as the display device illustrated in FIGS. 9A and 9B but isdifferent from the display device of Embodiment 1 in that the signalprocessing unit 100 is provided in a depressed portion 1010 provided inthe substrate 760, and the circuit layer 110 is electrically connectedto the pixel portion 762 in the depressed portion 1010. Preferably, thesurface of the substrate 760 (also referred to as a side where the pixelportion 762 is formed) is aligned or substantially aligned with thebottom surface of the base substrate 115 (also referred to as a sidewhere the circuit layer 110 is not formed), as illustrated in FIG. 11B.

The depressed portion 1010 can be formed by, for example, processing thesubstrate 760 using any of a variety of cutting machines or laserirradiation. Alternatively, the depressed portion 1010 may be obtainedwith a stacked layer structure of the substrate 760, as illustrated inFIG. 12.

Although FIG. 12 illustrates a two-layer structure of a substrate 760 aand a substrate 760 b, there is no limitation on the number of thelayers. The substrate 760 a and the substrate 760 b can be formed withany of the materials that can be used for the substrate 760, and thematerials of the substrate 760 a and the substrate 760 b may be the sameas or different from each other.

In the display device 750 described in Embodiment 3, the total thicknessof the signal processing unit 100 and the substrate 760 in a portionwhere the signal processing unit 100 is mounted and the thickness of thesubstrate 760 in a peripheral portion of the signal processing unit 100differ by the thickness of the signal processing unit 100. Hence, forexample, when the display device 750 is curved on the alternate long andshort dash line A1-A2 as an axis, the difference in thickness causes apartial variation in the deformation state (also referred to as thedegree of deformation) of the display device 750. This might locallyapply a force to part of the display device 750 or the conductivematerial 771 or reduce display quality of the pixel portion 762, forexample.

Further, when an external impact force is applied to the signalprocessing unit 100 (e.g., an object from above collides with the basesubstrate 115), the impact force easily concentrates on the signalprocessing unit 100 portion.

On the other hand, in the structure of the display device 1050 describedin this embodiment, the difference in thickness is reduced compared withthat in Embodiment 1, it is possible to solve the aforementionedproblems.

The methods and structures described in this embodiment can be combinedas appropriate with any of the methods and structures described in theother embodiments.

Embodiment 5

In this embodiment, a structure and a feature of a display device whichis partly different from the display devices described in the aboveembodiments are described. Note that although a manufacturing method isbasically similar to that in Embodiment 3, part of the method which isdifferent from that in Embodiment 3 is as appropriate described.

<Structure Example and Feature of Display Device>

A structure of a display device which is different from those in theabove embodiments is described below using FIGS. 13A to 13C. FIG. 13A isa top view of the display device, FIG. 13B is a cross-sectional viewtaken along the alternate long and short dash line E1-E2 in FIG. 11A,and FIG. 13C is a cross-sectional view taken along the alternate longand short dash line F1-F2 in FIG. 13A.

A display device 1250 illustrated in FIGS. 13A to 13C is different fromthe display devices in the above embodiments in that a protectivesubstrate 1210 is provided on the periphery of the signal processingunit 100. Preferably, the surface of the protective substrate 1210 (alsoreferred to as the side opposite to the side facing the substrate 760)is aligned or substantially aligned with the surface of the basesubstrate 115 (also referred to as a side where the circuit layer 110 isnot formed), as illustrated in FIG. 13B.

As the protective substrate 1210, any of the flexible substrates thatcan be used for the first temporary fixing substrate 314 and the basesubstrate 115 can be used to be bonded to the substrate 760 using anadhesive material (e.g., an adhesive or an adhesive tape). When thesubstrate 760 is formed using a flexible substrate, the protectivesubstrate 1210 is preferably formed using any of the materials that canbe used for the base substrate 115.

In the case where the depressed portion 1010 is formed in the substrate760 as in Embodiment 4, long time taken for the formation of thedepressed portion 1010 or a reduction in the intensity of the substrate760 is possible depending on the processing method. However, in the casewhere the protective substrate 1210 is provided in the peripheralportion of the signal processing unit 100 as in this embodiment, theprotective substrate 1210 can be bonded to the substrate 760 in a shorttime.

The methods and structures described in this embodiment can be combinedas appropriate with any of the methods and structures described in theother embodiments.

Embodiment 6

In this embodiment, a structure and a feature of a display device whichis partly different from the display devices described in the aboveembodiments are described. Note that although a manufacturing method isbasically similar to that in Embodiment 3, part of the method which isdifferent from that in Embodiment 3 is as appropriate described.

<Structure Example and Feature of Display Device>

A structure of a display device which is different from those in theabove embodiments is described using FIGS. 14A to 14C. FIG. 14A is a topview of the display device, FIG. 14B is a cross-sectional view takenalong the alternate long and short dash line G1-G2 in FIG. 11A, and FIG.14C is a cross-sectional view taken along the alternate long and shortdash line H1-H2 in FIG. 14A.

A display device 1350 illustrated in FIGS. 14A to 14C is different fromthe display devices in the above embodiments in that the base substrate115 used for the signal processing unit 100 also serves as the countersubstrate 770.

In order to allow the base substrate 115 to serve as the countersubstrate 770, at the time of the bonding of the circuit layer 110 tothe base substrate 115 described in Embodiment 1 referring to FIG. 8A,the base substrate 115 having a sufficiently larger area than thecircuit layer 110 is bonded to the circuit layer 110 with the bondmaterial 604 as illustrated in FIG. 15.

In this embodiment, the mounting of the signal processing unit 100 andthe bonding of the counter substrate can be performed on the substrate760 at the same time, so that the light-emitting device can bemanufactured in a shorter time. Further, the height of the portion wherethe signal processing unit 100 is mounted can be the same orsubstantially the same as the height of the other portions (theperiphery of the signal processing unit 100, the pixel portion 762, andthe like), so that local application of a force to part of the displaydevice 1350 or the conductive material 771, a reduction in displayquality, concentration of an impact force, or the like can besuppressed.

Note that a depressed portion may be provided in part of the substrate760 so that the signal processing unit 100 is mounted on the depressedportion, as described in Embodiment 4.

The methods and structures described in this embodiment can be combinedas appropriate with any of the methods and structures described in theother embodiments.

Embodiment 7

Examples of electronic devices are a television set (also referred to asa television or a television receiver), a monitor of a computer or thelike, a camera such as a digital camera or a digital video camera, adigital photo frame, a mobile phone handset (also referred to as amobile phone or a mobile phone device), a portable game machine, aportable information terminal, an audio reproducing device, alarge-sized game machine such as a pachinko machine, and the like.Examples of electronic devices each including the semiconductor devicedescribed in the above embodiment will be described.

FIG. 16A illustrates a display. A display 1601 includes a supportingbase 1602 and a display portion 1603. The display portion 1603 may beformed using a flexible substrate, which can realize a lightweight andthin display. Further, the display portion 1603 can be bent, and can bedetached from the supporting base 1602 and the display can be mountedalong a curved wall. A flexible display, which is one embodiment of thepresent invention, can be manufactured with the use of the semiconductordevice described in the above embodiment for the display portion 1603.Thus, the flexible display can be provided on a curved portion as wellas a flat surface; therefore, it can be used for various applications.

FIG. 16B illustrates a display capable of being wound. A display 1611includes a display portion 1612. A thin and large-area display, which isone embodiment of the present invention, can be manufactured with theuse of the semiconductor device described in the above embodiment forthe display portion 1612. Since the display 1611 may be formed using aflexible substrate, the display 1611 can be carried by being bent orwound along with the display portion 1612 even in the case where thedisplay 1611 is large.

FIG. 16C illustrates a sheet-type computer. A computer 1621 includes adisplay portion 1622, a keyboard 1623, a touch pad 1624, an externalconnection port 1625, a power plug 1626, and the like. A thin orsheet-type computer can be manufactured, which is one embodiment of thepresent invention, with the use of the semiconductor device described inthe above embodiment for the display portion 1622. The display portion1622 may be formed using a flexible substrate, which can realize alightweight and thin computer. Further, the display portion 1622 can bewound and stored in a main body when the main body of the computer 1621is provided with a storage space. Furthermore, by also forming thekeyboard 1623 to be flexible, the keyboard 1623 can be wound and storedin the storage space of the main body in a manner similar to that of thedisplay portion 1622 when it is not used, which is convenient forcarrying around.

FIG. 16D illustrates a display device having a 20 to 100-inchlarge-sized display portion. A display device 1631 includes a keyboard1633 which is an operation portion, a display portion 1632, a speaker1634, and the like. Since the display portion 1632 may be formed using aflexible substrate, the display device 1631 can be carried by being bentor wound with the keyboard 1633 detached. Further, the keyboard 1633 andthe display portion 1632 can be connected wirelessly. For example, thedisplay device 1631 can be mounted along a curved wall and can beoperated with the keyboard 1633 wirelessly.

In the example in FIG. 16D, the semiconductor device described in theabove embodiment is used for the display portion 1632. Thus, a thin andlarge-area display device can be manufactured, which is one embodimentof the present invention.

FIG. 16E shows an electronic book. An electronic book 1641 includes adisplay portion 1642, an operating key 1643, and the like. In addition,a modem may be incorporated in the electronic book 1641. The displayportion 1642 may be formed using a flexible substrate and can be bent orwound. Therefore, the electronic book can also be carried withoutoccupying a space. Further, the display portion 1642 can display amoving image as well as a still image such as a character.

In the example in FIG. 16E, the semiconductor device described in theabove embodiment is used for the display portion 1642. Thus, a thinelectronic book can be manufactured, which is one embodiment of thepresent invention.

FIG. 16F illustrates an IC card. An IC card 1651 includes a displayportion 1652, a connection terminal 1653, and the like. Since thedisplay portion 1652 may be formed using a flexible substrate to have alightweight and thin sheet-like shape, it can be attached onto a cardsurface. When the IC card can receive data without contact, informationobtained from outside can be displayed on the display portion 1652.

In the example in FIG. 16F, the semiconductor device described in theabove embodiment is used for the display portion 1652. Thus, a thin ICcard can be manufactured, which is one embodiment of the presentinvention.

When a semiconductor device according to one embodiment of the presentinvention is used for an electronic device, even in the case whereexternal force such as that caused in bending is applied to theelectronic device, embrittlement of an element such as a transistor canbe suppressed; thus, yield and reliability of the semiconductor devicecan be increased.

As described above, the application range of the present invention is sowide that the present invention can be applied to electronic devices andinformation displaying means in a wide variety of fields.

This embodiment can be combined with any of the other embodiments asappropriate.

This application is based on Japanese Patent Application serial No.2012-074335 filed with the Japan Patent Office on Mar. 28, 2012, theentire contents of which are hereby incorporated by reference.

What is claimed is:
 1. A signal processing unit comprising: a shiftregister circuit; a switching circuit; and a latch circuit which islocated over the shift register circuit and the switching circuit,wherein the latch circuit overlaps with the shift register circuit andthe switching circuit.
 2. The signal processing unit according to claim1, wherein each of the shift register circuit and the switching circuitcomprises a transistor whose channel formation region comprises silicon.3. The signal processing unit according to claim 2, wherein the channelformation region comprises single crystal silicon.
 4. The signalprocessing unit according to claim 1, wherein the latch circuitcomprises a transistor whose channel formation region comprises an oxidesemiconductor.
 5. The signal processing unit according to claim 1,wherein the latch circuit comprises a first transistor and a secondtransistor, and wherein one of a source and a drain of the firsttransistor is electrically connected to one of a source and a drain ofthe second transistor.
 6. The signal processing unit according to claim5, further comprising a first capacitor, wherein one of electrodes ofthe first capacitor is electrically connected to the one of the sourceand the drain of the first transistor and the one of the source and thedrain of the second transistor.
 7. The signal processing unit accordingto claim 5, further comprising a second capacitor, wherein one ofelectrodes of the second capacitor is electrically connected to theother of the source and the drain of the second transistor.
 8. Thesignal processing unit according to claim 5, wherein each of the firsttransistor and the second transistor comprises a channel formationregion which comprises an oxide semiconductor.
 9. A display devicecomprising: a substrate; a pixel portion over the substrate, the pixelportion including a thin film transistor and a pixel electrode; adisplay element layer over the pixel portion; a counter substrate overthe display element layer; and a signal processing unit electricallyconnected to the pixel portion, wherein the signal processing unitcomprises a shift register circuit, a switching circuit, and a latchcircuit, wherein the latch circuit is located over the shift registercircuit and the switching circuit, and wherein the latch circuitoverlaps with the shift register circuit and the switching circuit. 10.The display device according to claim 9, wherein each of the shiftregister circuit and the switching circuit comprises a transistor whosechannel formation region comprises silicon.
 11. The display deviceaccording to claim 10, wherein the channel formation region comprisessingle crystal silicon.
 12. The display device according to claim 9,wherein the latch circuit comprises a transistor whose channel formationregion comprises an oxide semiconductor.
 13. The display deviceaccording to claim 9, wherein the latch circuit comprises a firsttransistor and a second transistor, and wherein one of a source and adrain of the first transistor is electrically connected to one of asource and a drain of the second transistor.
 14. The display deviceaccording to claim 13, further comprising a first capacitor, wherein oneof electrodes of the first capacitor is electrically connected to theone of the source and the drain of the first transistor and the one ofthe source and the drain of the second transistor.
 15. The displaydevice according to claim 13, further comprising a second capacitor,wherein one of electrodes of the second capacitor is electricallyconnected to the other of the source and the drain of the secondtransistor.
 16. The display device according to claim 13, wherein eachof the first transistor and the second transistor comprises a channelformation region which comprises an oxide semiconductor.
 17. A methodfor manufacturing a signal processing unit, the method comprising thesteps of: forming a first semiconductor film over a substrate; forming ashift register and a switching circuit using the first semiconductorfilm; forming a second semiconductor film over the shift register andthe switching circuit; and forming a latch circuit using the secondsemiconductor film.
 18. The method according to claim 17, wherein thelatch circuit is formed so as to overlap with the shift register and theswitching circuit.
 19. The method according to claim 17, wherein thefirst semiconductor film comprises silicon.
 20. The method according toclaim 17, wherein the second semiconductor film comprises an oxidesemiconductor.